Fix @deffn without category.

Fix @deffn without category.

* abbrevs.texi (Abbrev Expansion): Replace @deffn with @defun
for `abbrev-insert'.

* buffers.texi (Indirect Buffers): Add category `Command'
to @deffn of `clone-indirect-buffer'.

* windows.texi (Cyclic Window Ordering): Replace @deffn with @defun
for `next-window' and `previous-window'.  Add category `Command'
to @deffn of `pop-to-buffer'.