org.texi: remove footnote from subheading

* doc/org.texi (Agenda commands): Remove footnote from
  @tsubheading (as a macro argument it would need to be kept on a
  single line) and add a sentence with the reference instead.

doc/org.texi

@@ -8713,9 +8713,10 @@ that entry would be in the original buffer (taken from a property, from a
 Remove the restriction lock on the agenda, if it is currently restricted to a
 file or subtree (@pxref{Agenda files}).
 
-@tsubheading{Secondary filtering and query editing@footnote{See
+@tsubheading{Secondary filtering and query editing}
-@pxref{Filtering/limiting agenda items} for a detailed description of these
+
-commands.}}
+For a detailed description of these commands, see @pxref{Filtering/limiting
+agenda items}.
 
 @orgcmd{/,org-agenda-filter-by-tag}
 @vindex org-agenda-tag-filter-preset