DOCS/man/input: reserve user-data/osc and user-data/mpv sub-paths

Currently user-data/osc is used for interpolation between the osc and
other internal scripts. Reserve this sub-path and also user-data/mpv
to make sure external scripts can only use these values as directed.
New internal uses of user-data should use user-data/mpv instead.

DOCS/interface-changes/user-data-subpath.txt

@@ -0,0 +1 @@
+reserve `user-data/osc` and `user-data/mpv` sub-paths for internal use

DOCS/man/input.rst

@@ -3549,6 +3549,12 @@ Property list
     The player itself does not use any data in it (although some builtin scripts may).
     The property is not preserved across player restarts.
 
+    The following sub-paths are reserved for internal uses or have special semantics:
+    ``user-data/osc``, ``user-data/mpv``. Unless noted otherwise, the semantics of
+    any properties under these sub-paths can change at any time and may not be relied
+    upon, and writing to these properties may prevent builtin scripts from working
+    properly.
+
     Sub-paths can be accessed directly; e.g. ``user-data/my-script/state/a`` can be
     read, written, or observed.