runtime: remove redundant locking in mcache.refill

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Also, mcache.refill's documentation is generally a bit out-of-date, so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