bytes: API tweaks - fix documentation for NewBuffer and NewBufferString - document and implement behavior of Truncate on invalid lengths
bytes: API tweaks
- fix documentation for NewBuffer and NewBufferString
- document and implement behavior of Truncate on invalid lengths

Fixes issue 2837.

R=rsc, adg
CC=golang-dev
http://codereview.appspot.com/5637044

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go