bytes.Buffer: remove dead code, complete documentation
bytes.Buffer: remove dead code, complete documentation

R=r, dave, r
CC=golang-dev
http://codereview.appspot.com/5533086

tv42/old-go,dsriram/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,gnanderson/go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,dsriram/golang,dsriram/golang,gnanderson/go,dsriram/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go