os: recomment MkdirAll
os: recomment MkdirAll

The internal comments are not completely precise about
what is going on, and they are causing confusion.

Fixes issue 8283.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/151460043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go