reflect: match MakeMapWithSize docs about initial capacity with spec

reflect: match MakeMapWithSize docs about initial capacity with spec

Following the spec clarification in CL 40393, copy that text
to reflect docs to state that the initial capacity of MakeMapWithSize
is a hint/approximate.

Fixes #19903

Change-Id: I6b3315b8183cafaa61fbb2839a4e42b76fd71544
Reviewed-on: https://go-review.googlesource.com/46270
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>