Mention the mx capability a couple of times to be a little clearer.
Two contributors submitted PRs about the same problem in different
parts of the doc.

PR: docs/12148
Submitted by: Yoshihiro Koya <koya@math.yokohama-cu.ac.jp>
Reviewed by: Sean Kelly <kelly@ad1440.net>

PR: docs/12284
Submitted by: Panagiotis Astithas <past@netmode.ntua.gr>
Reviewed by: nik