doc: be more precise about \b
doc: be more precise about \b

R=rsc
CC=re2-dev
http://codereview.appspot.com/4119045

AtnNn/re2,driskell/re2-1,google/re2,Cfretz244/re2,evilmucedin/re2,google/re2,simudream/re2,bowlofstew/re2,Cfretz244/re2,evilmucedin/re2,AtnNn/re2,rAntonioh/re2,pombredanne/re2,bowlofstew/re2,ElectronicRU/re2,yanqingmen/re2,pombredanne/re2,wanggchongg/re2,AtnNn/re2,ElectronicRU/re2,yanqingmen/re2,google/re2,yanqingmen/re2,pombredanne/re2,evilmucedin/re2,rAntonioh/re2,AtnNn/re2,lnussbaum/re2,pombredanne/re2,google/re2,yanqingmen/re2,rAntonioh/re2,pombredanne/re2,ElectronicRU/re2,bowlofstew/re2,lnussbaum/re2,driskell/re2-1,rAntonioh/re2,AtnNn/re2,rAntonioh/re2,Cfretz244/re2,simudream/re2,yanqingmen/re2,driskell/re2-1,wanggchongg/re2,evilmucedin/re2,wanggchongg/re2,simudream/re2,google/re2,driskell/re2-1,simudream/re2,ElectronicRU/re2,simudream/re2,ElectronicRU/re2,Cfretz244/re2,bowlofstew/re2,bowlofstew/re2,lnussbaum/re2,driskell/re2-1,evilmucedin/re2,wanggchongg/re2,Cfretz244/re2,lnussbaum/re2,lnussbaum/re2,wanggchongg/re2