str: Correct documentation on is_char_boundary
str: Correct documentation on is_char_boundary

Documentation claims it panics on out of bounds -- it regards out of
bounds as just not a char boundary.

core::str module is aware of how it works and uses it appropriately.
Maybe we should rename it to `is_valid_index`, `is_slicable_index`, or
something similar.

jroesch/rust,zubron/rust,zubron/rust,reem/rust,GBGamer/rust,graydon/rust,jashank/rust,zubron/rust,mvdnes/rust,graydon/rust,l0kod/rust,vhbit/rust,GBGamer/rust,pshc/rust,nwin/rust,dwillmer/rust,vhbit/rust,nwin/rust,GBGamer/rust,mvdnes/rust,mvdnes/rust,pshc/rust,nwin/rust,reem/rust,mvdnes/rust,l0kod/rust,jashank/rust,reem/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,jroesch/rust,pshc/rust,pshc/rust,jroesch/rust,jashank/rust,l0kod/rust,GBGamer/rust,jashank/rust,GBGamer/rust,vhbit/rust,nwin/rust,dwillmer/rust,jroesch/rust,reem/rust,l0kod/rust,nwin/rust,l0kod/rust,nwin/rust,aidancully/rust,l0kod/rust,zubron/rust,jroesch/rust,reem/rust,zubron/rust,vhbit/rust,graydon/rust,aidancully/rust,jroesch/rust,jroesch/rust,zubron/rust,jashank/rust,dwillmer/rust,mvdnes/rust,jashank/rust,pshc/rust,aidancully/rust,aidancully/rust,dwillmer/rust,pshc/rust,mvdnes/rust,reem/rust,vhbit/rust,dwillmer/rust,pshc/rust,zubron/rust,graydon/rust,aidancully/rust,graydon/rust,l0kod/rust,zubron/rust,jroesch/rust,jashank/rust,dwillmer/rust,aidancully/rust,vhbit/rust,dwillmer/rust,nwin/rust,jashank/rust,vhbit/rust,vhbit/rust,l0kod/rust,nwin/rust,dwillmer/rust,pshc/rust