Better comment for inplace_update_support
Better comment for inplace_update_support

Summary: See https://github.com/facebook/rocksdb/issues/215

Test Plan: none

Reviewers: dhruba, sdong, ljin, yhchiang, nkg-

Reviewed By: nkg-

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20769

JackLian/rocksdb,Vaisman/rocksdb,anagav/rocksdb,luckywhu/rocksdb,Applied-Duality/rocksdb,amyvmiwei/rocksdb,ylong/rocksdb,NickCis/rocksdb,rDSN-Projects/rocksdb.replicated,wskplho/rocksdb,NickCis/rocksdb,ryneli/rocksdb,virtdb/rocksdb,JoeWoo/rocksdb,Applied-Duality/rocksdb,wat-ze-hex/rocksdb,geraldoandradee/rocksdb,Andymic/rocksdb,jalexanderqed/rocksdb,temicai/rocksdb,tizzybec/rocksdb,tsheasha/rocksdb,bbiao/rocksdb,anagav/rocksdb,JoeWoo/rocksdb,wenduo/rocksdb,jalexanderqed/rocksdb,vashstorm/rocksdb,dkorolev/rocksdb,tsheasha/rocksdb,ylong/rocksdb,siddhartharay007/rocksdb,hobinyoon/rocksdb,luckywhu/rocksdb,anagav/rocksdb,msb-at-yahoo/rocksdb,wenduo/rocksdb,wskplho/rocksdb,bbiao/rocksdb,wskplho/rocksdb,vmx/rocksdb,facebook/rocksdb,IMCG/RcoksDB,rDSN-Projects/rocksdb.replicated,wenduo/rocksdb,OverlordQ/rocksdb,zhangpng/rocksdb,OverlordQ/rocksdb,Andymic/rocksdb,dkorolev/rocksdb,lgscofield/rocksdb,vmx/rocksdb,Applied-Duality/rocksdb,vmx/rocksdb,tschottdorf/rocksdb,geraldoandradee/rocksdb,Vaisman/rocksdb,RyanTech/rocksdb,NickCis/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,norton/rocksdb,lgscofield/rocksdb,luckywhu/rocksdb,hobinyoon/rocksdb,RyanTech/rocksdb,zhangpng/rocksdb,JoeWoo/rocksdb,kaschaeffer/rocksdb,ylong/rocksdb,luckywhu/rocksdb,makelivedotnet/rocksdb,facebook/rocksdb,facebook/rocksdb,sorphi/rocksdb,JackLian/rocksdb,wat-ze-hex/rocksdb,Vaisman/rocksdb,norton/rocksdb,biddyweb/rocksdb,wskplho/rocksdb,wskplho/rocksdb,tizzybec/rocksdb,virtdb/rocksdb,facebook/rocksdb,norton/rocksdb,ylong/rocksdb,JackLian/rocksdb,tsheasha/rocksdb,facebook/rocksdb,SunguckLee/RocksDB,msb-at-yahoo/rocksdb,vashstorm/rocksdb,norton/rocksdb,JoeWoo/rocksdb,makelivedotnet/rocksdb,RyanTech/rocksdb,vashstorm/rocksdb,geraldoandradee/rocksdb,caijieming-baidu/rocksdb,jalexanderqed/rocksdb,skunkwerks/rocksdb,jalexanderqed/rocksdb,fengshao0907/rocksdb,OverlordQ/rocksdb,amyvmiwei/rocksdb,sorphi/rocksdb,amyvmiwei/rocksdb,makelivedotnet/rocksdb,jalexanderqed/rocksdb,biddyweb/rocksdb,dkorolev/rocksdb,wlqGit/rocksdb,JohnPJenkins/rocksdb,luckywhu/rocksdb,OverlordQ/rocksdb,mbarbon/rocksdb,siddhartharay007/rocksdb,sorphi/rocksdb,wenduo/rocksdb,zhangpng/rocksdb,OverlordQ/rocksdb,hobinyoon/rocksdb,temicai/rocksdb,biddyweb/rocksdb,JoeWoo/rocksdb,bbiao/rocksdb,kaschaeffer/rocksdb,sorphi/rocksdb,skunkwerks/rocksdb,virtdb/rocksdb,makelivedotnet/rocksdb,rDSN-Projects/rocksdb.replicated,luckywhu/rocksdb,tschottdorf/rocksdb,caijieming-baidu/rocksdb,amyvmiwei/rocksdb,lgscofield/rocksdb,wenduo/rocksdb,makelivedotnet/rocksdb,IMCG/RcoksDB,Andymic/rocksdb,wlqGit/rocksdb,kaschaeffer/rocksdb,anagav/rocksdb,rDSN-Projects/rocksdb.replicated,ryneli/rocksdb,geraldoandradee/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,bbiao/rocksdb,alihalabyah/rocksdb,bbiao/rocksdb,skunkwerks/rocksdb,JackLian/rocksdb,wlqGit/rocksdb,hobinyoon/rocksdb,msb-at-yahoo/rocksdb,JoeWoo/rocksdb,hobinyoon/rocksdb,temicai/rocksdb,wlqGit/rocksdb,tschottdorf/rocksdb,JohnPJenkins/rocksdb,amyvmiwei/rocksdb,facebook/rocksdb,tizzybec/rocksdb,jalexanderqed/rocksdb,vmx/rocksdb,anagav/rocksdb,skunkwerks/rocksdb,Applied-Duality/rocksdb,ryneli/rocksdb,vashstorm/rocksdb,vmx/rocksdb,geraldoandradee/rocksdb,temicai/rocksdb,temicai/rocksdb,tizzybec/rocksdb,amyvmiwei/rocksdb,rDSN-Projects/rocksdb.replicated,Vaisman/rocksdb,temicai/rocksdb,NickCis/rocksdb,Applied-Duality/rocksdb,Applied-Duality/rocksdb,RyanTech/rocksdb,vmx/rocksdb,vashstorm/rocksdb,mbarbon/rocksdb,caijieming-baidu/rocksdb,alihalabyah/rocksdb,JohnPJenkins/rocksdb,wat-ze-hex/rocksdb,siddhartharay007/rocksdb,IMCG/RcoksDB,ryneli/rocksdb,RyanTech/rocksdb,flabby/rocksdb,RyanTech/rocksdb,wskplho/rocksdb,caijieming-baidu/rocksdb,kaschaeffer/rocksdb,JohnPJenkins/rocksdb,wlqGit/rocksdb,JohnPJenkins/rocksdb,mbarbon/rocksdb,OverlordQ/rocksdb,tschottdorf/rocksdb,jalexanderqed/rocksdb,fengshao0907/rocksdb,zhangpng/rocksdb,msb-at-yahoo/rocksdb,tschottdorf/rocksdb,NickCis/rocksdb,IMCG/RcoksDB,fengshao0907/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,JackLian/rocksdb,wenduo/rocksdb,JohnPJenkins/rocksdb,NickCis/rocksdb,luckywhu/rocksdb,tsheasha/rocksdb,msb-at-yahoo/rocksdb,flabby/rocksdb,flabby/rocksdb,vmx/rocksdb,dkorolev/rocksdb,geraldoandradee/rocksdb,JackLian/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,anagav/rocksdb,skunkwerks/rocksdb,bbiao/rocksdb,tsheasha/rocksdb,JoeWoo/rocksdb,biddyweb/rocksdb,biddyweb/rocksdb,fengshao0907/rocksdb,siddhartharay007/rocksdb,wat-ze-hex/rocksdb,kaschaeffer/rocksdb,tschottdorf/rocksdb,Vaisman/rocksdb,alihalabyah/rocksdb,NickCis/rocksdb,wlqGit/rocksdb,caijieming-baidu/rocksdb,flabby/rocksdb,flabby/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,JohnPJenkins/rocksdb,fengshao0907/rocksdb,lgscofield/rocksdb,sorphi/rocksdb,vashstorm/rocksdb,wenduo/rocksdb,amyvmiwei/rocksdb,IMCG/RcoksDB,JackLian/rocksdb,fengshao0907/rocksdb,IMCG/RcoksDB,geraldoandradee/rocksdb,virtdb/rocksdb,ylong/rocksdb,siddhartharay007/rocksdb,tizzybec/rocksdb,makelivedotnet/rocksdb,hobinyoon/rocksdb,lgscofield/rocksdb,alihalabyah/rocksdb,ryneli/rocksdb,sorphi/rocksdb,RyanTech/rocksdb,caijieming-baidu/rocksdb,alihalabyah/rocksdb,SunguckLee/RocksDB,rDSN-Projects/rocksdb.replicated,skunkwerks/rocksdb,zhangpng/rocksdb,temicai/rocksdb,hobinyoon/rocksdb,SunguckLee/RocksDB,wskplho/rocksdb,norton/rocksdb,wat-ze-hex/rocksdb,norton/rocksdb,ylong/rocksdb,rDSN-Projects/rocksdb.replicated,facebook/rocksdb,ylong/rocksdb,tsheasha/rocksdb,skunkwerks/rocksdb,dkorolev/rocksdb,norton/rocksdb,dkorolev/rocksdb,alihalabyah/rocksdb,virtdb/rocksdb,Andymic/rocksdb,dkorolev/rocksdb,anagav/rocksdb,mbarbon/rocksdb,vmx/rocksdb,ryneli/rocksdb,wlqGit/rocksdb,norton/rocksdb,Applied-Duality/rocksdb,flabby/rocksdb,alihalabyah/rocksdb,msb-at-yahoo/rocksdb,tizzybec/rocksdb,lgscofield/rocksdb,wat-ze-hex/rocksdb,hobinyoon/rocksdb,kaschaeffer/rocksdb,caijieming-baidu/rocksdb,biddyweb/rocksdb,virtdb/rocksdb,Andymic/rocksdb,siddhartharay007/rocksdb,ryneli/rocksdb,tizzybec/rocksdb,zhangpng/rocksdb,wat-ze-hex/rocksdb,vashstorm/rocksdb,Andymic/rocksdb,mbarbon/rocksdb,SunguckLee/RocksDB,lgscofield/rocksdb,makelivedotnet/rocksdb,zhangpng/rocksdb,tsheasha/rocksdb,tsheasha/rocksdb,facebook/rocksdb,mbarbon/rocksdb,fengshao0907/rocksdb,sorphi/rocksdb,virtdb/rocksdb,Vaisman/rocksdb,IMCG/RcoksDB,biddyweb/rocksdb,mbarbon/rocksdb,siddhartharay007/rocksdb,kaschaeffer/rocksdb,wenduo/rocksdb,Vaisman/rocksdb