[release-branch.go1.12] doc/go1.12: soften, expand crypto/rc4 assembly removal text

Change-Id: I46fa43f6c5ac49386f4622e1363d8976f49c0894
Reviewed-on: https://go-review.googlesource.com/c/162019


Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit c75ee696)
Reviewed-on: https://go-review.googlesource.com/c/162357