manpage: mention the client API/interface change logs

Also, I'm seeing that we still have mplayer-changes.rst - add a warning
that it's outdated.

DOCS/man/changes.rst

@@ -8,6 +8,11 @@ There is no real changelog, but you can look at the following things:
 
   https://github.com/mpv-player/mpv/releases
 * The git log, which is the "real" changelog
+* The files ``client-api-changes.rst`` and ``interface-changes.rst`` in the
+  ``DOCS`` sub directoryon the git repository, which document API and user
+  interface changes (the latter usually documents breaking changes only, rather
+  than additions).
 * The file ``mplayer-changes.rst`` in the ``DOCS`` sub directory on the git
   repository, which used to be in place of this section. It documents some
-  changes that happened since mplayer2 forked off MPlayer.
+  changes that happened since mplayer2 forked off MPlayer. (Not updated
+  anymore.)

DOCS/mplayer-changes.rst

@@ -7,8 +7,8 @@ large part of which is incompatible or completely changes how the player
 behaves. Although there are still many similarities to its ancestors, **mpv**
 should generally be treated as a completely different program.
 
-.. note::
-    These lists are incomplete.
+.. admonition:: Warning
+    This document is not updated anymore, and is incomplete and outdated.
 
 General Changes from MPlayer to mpv
 -----------------------------------