Update --help message in db_bench.
Update --help message in db_bench.

Summary:
Remove --help entry for readhot.

Update read_random_exp_range flag description: The distribution is num *
exp(-r), not num * exp(r).

Test Plan: Run ./db_bench --help

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D42303

makelivedotnet/rocksdb,lgscofield/rocksdb,wlqGit/rocksdb,zhangpng/rocksdb,luckywhu/rocksdb,RyanTech/rocksdb,kaschaeffer/rocksdb,caijieming-baidu/rocksdb,wenduo/rocksdb,caijieming-baidu/rocksdb,ylong/rocksdb,norton/rocksdb,biddyweb/rocksdb,temicai/rocksdb,ylong/rocksdb,norton/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,JoeWoo/rocksdb,dkorolev/rocksdb,ryneli/rocksdb,tsheasha/rocksdb,ryneli/rocksdb,ryneli/rocksdb,dkorolev/rocksdb,skunkwerks/rocksdb,skunkwerks/rocksdb,RyanTech/rocksdb,makelivedotnet/rocksdb,tsheasha/rocksdb,ryneli/rocksdb,jalexanderqed/rocksdb,dkorolev/rocksdb,kaschaeffer/rocksdb,geraldoandradee/rocksdb,zhangpng/rocksdb,alihalabyah/rocksdb,fengshao0907/rocksdb,alihalabyah/rocksdb,Vaisman/rocksdb,makelivedotnet/rocksdb,wat-ze-hex/rocksdb,facebook/rocksdb,jalexanderqed/rocksdb,Applied-Duality/rocksdb,mbarbon/rocksdb,jalexanderqed/rocksdb,RyanTech/rocksdb,bbiao/rocksdb,norton/rocksdb,ryneli/rocksdb,JackLian/rocksdb,vashstorm/rocksdb,Andymic/rocksdb,JohnPJenkins/rocksdb,OverlordQ/rocksdb,facebook/rocksdb,flabby/rocksdb,bbiao/rocksdb,Vaisman/rocksdb,lgscofield/rocksdb,norton/rocksdb,alihalabyah/rocksdb,hobinyoon/rocksdb,vashstorm/rocksdb,luckywhu/rocksdb,makelivedotnet/rocksdb,biddyweb/rocksdb,luckywhu/rocksdb,JoeWoo/rocksdb,SunguckLee/RocksDB,biddyweb/rocksdb,fengshao0907/rocksdb,JoeWoo/rocksdb,sorphi/rocksdb,bbiao/rocksdb,JohnPJenkins/rocksdb,tsheasha/rocksdb,Andymic/rocksdb,bbiao/rocksdb,zhangpng/rocksdb,vashstorm/rocksdb,SunguckLee/RocksDB,anagav/rocksdb,hobinyoon/rocksdb,alihalabyah/rocksdb,kaschaeffer/rocksdb,caijieming-baidu/rocksdb,alihalabyah/rocksdb,vmx/rocksdb,hobinyoon/rocksdb,tizzybec/rocksdb,geraldoandradee/rocksdb,sorphi/rocksdb,sorphi/rocksdb,luckywhu/rocksdb,sorphi/rocksdb,tizzybec/rocksdb,hobinyoon/rocksdb,wenduo/rocksdb,ylong/rocksdb,lgscofield/rocksdb,fengshao0907/rocksdb,sorphi/rocksdb,Vaisman/rocksdb,lgscofield/rocksdb,anagav/rocksdb,SunguckLee/RocksDB,luckywhu/rocksdb,tsheasha/rocksdb,norton/rocksdb,lgscofield/rocksdb,luckywhu/rocksdb,temicai/rocksdb,wskplho/rocksdb,wlqGit/rocksdb,geraldoandradee/rocksdb,norton/rocksdb,ylong/rocksdb,OverlordQ/rocksdb,fengshao0907/rocksdb,anagav/rocksdb,hobinyoon/rocksdb,wenduo/rocksdb,ylong/rocksdb,biddyweb/rocksdb,makelivedotnet/rocksdb,Applied-Duality/rocksdb,JohnPJenkins/rocksdb,JohnPJenkins/rocksdb,Vaisman/rocksdb,jalexanderqed/rocksdb,dkorolev/rocksdb,fengshao0907/rocksdb,skunkwerks/rocksdb,SunguckLee/RocksDB,wenduo/rocksdb,caijieming-baidu/rocksdb,JackLian/rocksdb,facebook/rocksdb,fengshao0907/rocksdb,dkorolev/rocksdb,temicai/rocksdb,tsheasha/rocksdb,facebook/rocksdb,facebook/rocksdb,tsheasha/rocksdb,flabby/rocksdb,temicai/rocksdb,Andymic/rocksdb,vashstorm/rocksdb,mbarbon/rocksdb,vashstorm/rocksdb,jalexanderqed/rocksdb,JackLian/rocksdb,wat-ze-hex/rocksdb,zhangpng/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,temicai/rocksdb,vmx/rocksdb,Vaisman/rocksdb,skunkwerks/rocksdb,skunkwerks/rocksdb,caijieming-baidu/rocksdb,wat-ze-hex/rocksdb,wenduo/rocksdb,OverlordQ/rocksdb,sorphi/rocksdb,ylong/rocksdb,Vaisman/rocksdb,makelivedotnet/rocksdb,mbarbon/rocksdb,flabby/rocksdb,Applied-Duality/rocksdb,skunkwerks/rocksdb,JackLian/rocksdb,flabby/rocksdb,siddhartharay007/rocksdb,mbarbon/rocksdb,SunguckLee/RocksDB,tizzybec/rocksdb,vmx/rocksdb,Andymic/rocksdb,geraldoandradee/rocksdb,tsheasha/rocksdb,JoeWoo/rocksdb,wenduo/rocksdb,siddhartharay007/rocksdb,sorphi/rocksdb,wlqGit/rocksdb,JohnPJenkins/rocksdb,Andymic/rocksdb,flabby/rocksdb,kaschaeffer/rocksdb,zhangpng/rocksdb,geraldoandradee/rocksdb,JackLian/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,alihalabyah/rocksdb,luckywhu/rocksdb,wenduo/rocksdb,geraldoandradee/rocksdb,wlqGit/rocksdb,JoeWoo/rocksdb,JohnPJenkins/rocksdb,caijieming-baidu/rocksdb,wat-ze-hex/rocksdb,wlqGit/rocksdb,bbiao/rocksdb,temicai/rocksdb,hobinyoon/rocksdb,flabby/rocksdb,wlqGit/rocksdb,temicai/rocksdb,siddhartharay007/rocksdb,Vaisman/rocksdb,dkorolev/rocksdb,siddhartharay007/rocksdb,facebook/rocksdb,ylong/rocksdb,mbarbon/rocksdb,skunkwerks/rocksdb,OverlordQ/rocksdb,siddhartharay007/rocksdb,RyanTech/rocksdb,biddyweb/rocksdb,facebook/rocksdb,tizzybec/rocksdb,alihalabyah/rocksdb,bbiao/rocksdb,RyanTech/rocksdb,anagav/rocksdb,geraldoandradee/rocksdb,Andymic/rocksdb,makelivedotnet/rocksdb,JackLian/rocksdb,tizzybec/rocksdb,tizzybec/rocksdb,wskplho/rocksdb,siddhartharay007/rocksdb,Andymic/rocksdb,dkorolev/rocksdb,wskplho/rocksdb,biddyweb/rocksdb,vmx/rocksdb,tizzybec/rocksdb,mbarbon/rocksdb,hobinyoon/rocksdb,zhangpng/rocksdb,RyanTech/rocksdb,anagav/rocksdb,Applied-Duality/rocksdb,Applied-Duality/rocksdb,kaschaeffer/rocksdb,mbarbon/rocksdb,ryneli/rocksdb,hobinyoon/rocksdb,RyanTech/rocksdb,bbiao/rocksdb,lgscofield/rocksdb,anagav/rocksdb,facebook/rocksdb,kaschaeffer/rocksdb,lgscofield/rocksdb,vmx/rocksdb,zhangpng/rocksdb,flabby/rocksdb,ryneli/rocksdb,SunguckLee/RocksDB,OverlordQ/rocksdb,vashstorm/rocksdb,Applied-Duality/rocksdb,norton/rocksdb,wskplho/rocksdb,JackLian/rocksdb,Andymic/rocksdb,kaschaeffer/rocksdb,siddhartharay007/rocksdb,OverlordQ/rocksdb,vashstorm/rocksdb,vmx/rocksdb,vmx/rocksdb,JohnPJenkins/rocksdb,jalexanderqed/rocksdb,JoeWoo/rocksdb,wlqGit/rocksdb,anagav/rocksdb,fengshao0907/rocksdb,biddyweb/rocksdb,SunguckLee/RocksDB,caijieming-baidu/rocksdb,vmx/rocksdb,jalexanderqed/rocksdb,Applied-Duality/rocksdb,OverlordQ/rocksdb,wat-ze-hex/rocksdb,tsheasha/rocksdb,wskplho/rocksdb,wskplho/rocksdb,JoeWoo/rocksdb,norton/rocksdb,wskplho/rocksdb