doc: import clarifications from Go tree
doc: import clarifications from Go tree

TBR=rsc
CC=re2-dev
https://codereview.appspot.com/150610043

lamuguo/re2,dragonkid/re2,PiRSquared17/re2,zeusdeux/re2,lamuguo/re2,tst2005googlecode/re2,zeusdeux/re2,tst2005googlecode/re2,PiRSquared17/re2,tst2005googlecode/re2,dragonkid/re2,tst2005googlecode/re2,PiRSquared17/re2,dragonkid/re2,zeusdeux/re2,lamuguo/re2,zeusdeux/re2,lamuguo/re2,PiRSquared17/re2,pombreda/re2,tst2005googlecode/re2,zeusdeux/re2,lamuguo/re2,dragonkid/re2,pombreda/re2,pombreda/re2,pombreda/re2,dragonkid/re2,pombreda/re2,PiRSquared17/re2