The section on map literals mentions constant map keys but doesn't say
what happens for equal non-constant map keys - that is covered in the
section on evaluation order. Added respective link for clarity.

Fixes #19689.

Change-Id: If9a5368ba02e8250d4bb0a1d60d0de26a1f37bbb
Reviewed-on: https://go-review.googlesource.com/38598Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>