media-video/mpv: notify users about new OSC layout since 0.21.0 in 9999

media-video/mpv: notify users about new OSC layout since 0.21.0 in 9999

Nobody complained about new OSC layout since 0.21.0 (October 2016).
Thus don't bother ~arch users with it, but be nice to stable users.

Update softvol message to point to Gentoo wiki instead of bugzilla.
Gentoo mpv wiki page suffered a complete rewrite and became useful.
Version constraint at this point can be relaxed to just 0.18.1.

Package-Manager: Portage-2.3.3, Repoman-2.3.1
Closes: https://github.com/gentoo/gentoo/pull/3718