runtime: document MemStats

This documents all fields in MemStats and more clearly documents where
mstats differs from MemStats.

Fixes #15849.

Change-Id: Ie09374bcdb3a5fdd2d25fe4bba836aaae92cb1dd
Reviewed-on: https://go-review.googlesource.com/28972Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>