Some icomplete doc

Some icomplete doc

* doc/emacs/mini.texi (Completion Options): No longer mention icomplete,
which has its own section now.

* doc/emacs/modes.texi (Minor Modes): Update Icomplete xref.

* lisp/icomplete.el: Commentary updates.
(icomplete): Add info-link to defgroup.
(icomplete-with-completion-tables, icomplete-minibuffer-setup-hook)
(icomplete-minibuffer-map, icomplete-mode)
(icomplete-simple-completing-p, icomplete-completions): Doc fixes.

* etc/NEWS: Related edits.