etc/input.conf: add example how to change window size by key binding

Also drop a vague hint how to do it in the manpage.

DOCS/man/en/mpv.rst

@@ -204,6 +204,8 @@ corresponding adjustment, or the software equalizer (``--vf=eq``).)
 
 command + 0
     Resize movie window to half its original size.
+    (On other platforms, you can bind keys to change the ``window-scale``
+    property.)
 
 command + 1
     Resize movie window to its original size.

etc/input.conf

@@ -215,3 +215,4 @@
 # ? add sub-scale -0.1                  # decrease subtitle font size
 # ? sub_step -1                         # immediately display next subtitle
 # ? sub_step +1                         #                     previous
+# ? cycle_values window-scale 0.5 2 1   # switch between 1/2, 2x, unresized window size