manpage: deprecate input section commands

These were a bad idea and are obscure. Scripting key mapping support
still uses them, but this is not relevant to scripting authors, because
the mpv provided helper code (defaults.lua) takes care of this. In
addition, the OSC uses a legacy form of this.

Hopefully, this input section stuff can be removed, and replaced by a
simpler mechanism.

DOCS/interface-changes.rst

@@ -38,6 +38,8 @@ Interface changes
       but it will probably eventually go away)
     - deprecate --video-sync=display-adrop (might be removed if it's in the way;
       undeprecated or readded if it's not too much of a problem)
+    - deprecate all input section commands (these will be changed/removed, as
+      soon as mpv internals do not require them anymore)
  --- mpv 0.30.0 ---
     - add `--d3d11-output-format` to enable explicit selection of a D3D11
       swap chain format.

DOCS/man/input.rst

@@ -728,6 +728,8 @@ Input Commands that are Possibly Subject to Change
     list yourself when adding a second key binding for cycling backwards.
 
 ``enable-section <name> [<flags>]``
+    This command is deprecated, except for mpv-internal uses.
+
     Enable all key bindings in the named input section.
 
     The enabled input sections form a stack. Bindings in sections on the top of
@@ -750,9 +752,13 @@ Input Commands that are Possibly Subject to Change
         Same.
 
 ``disable-section <name>``
+    This command is deprecated, except for mpv-internal uses.
+
     Disable the named input section. Undoes ``enable-section``.
 
 ``define-section <name> <contents> [<flags>]``
+    This command is deprecated, except for mpv-internal uses.
+
     Create a named input section, or replace the contents of an already existing
     input section. The ``contents`` parameter uses the same syntax as the
     ``input.conf`` file (except that using the section syntax in it is not