sync: improve once.Do documentation readability

sync: improve once.Do documentation readability

The previous wording, though accurate, was hard to parse.
In particular, it was tempting to interpret "the method"
as referring to "the function f" instead of "Do", and
required effort to find the correct antecedent for
"this receiver".

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/13307043