systemd: Document future actions needed for set of musl patches

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 433d51c3)
Signed-off-by: Steve Sakoman <steve@sakoman.com>