manpage: generic notice about changed sub-option parsing

Just in case someone expects these are unchanged just because they're
not mentioned in changes.rst anywhere. Documenting all of these changes
would be too much work and not helpful either.

DOCS/man/en/changes.rst

@@ -185,6 +185,15 @@ Command Line Switches
 
     ``-opt val`` becomes ``--opt=val``.
 
+.. note::
+
+    Quite some video filters, video outputs, audio filters, audio outputs, had
+    changes in their option parsing. These aren't mentioned in the table above.
+
+    Also, some video and audio filters have been removed, and you have to use
+    libavfilter (using ``--vf=lavfi=[...]`` or ``--af=lavfi=[...]``) to get
+    them back.
+
 input.conf and Slave Commands
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~