mcache.refill acquires g.m.locks, which is pointless because the
caller itself absolutely must have done so already to prevent
ownership of mcache from shifting.

Also, mcache.refill's documentation is generally a bit out-of-date, so
this cleans this up.

Change-Id: Idc8de666fcaf3c3d96006bd23a8f307539587d6c
Reviewed-on: https://go-review.googlesource.com/138195
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>