patch 8.2.4498: using <Plug> with "noremap" does not work

Problem:    Using <Plug> with "noremap" does not work.
Solution:   Always remap <Plug>. (closes #9879, closes #9789)

runtime/doc/map.txt

@@ -80,6 +80,8 @@ modes.
 			where the map command applies.  Disallow mapping of
 			{rhs}, to avoid nested and recursive mappings.  Often
 			used to redefine a command.
+			Note: When <Plug> appears in the {rhs} this part is
+			always applied even if remapping is disallowed.
 
 
 :unm[ap]  {lhs}			|mapmode-nvo|		*:unm*  *:unmap*