Fix "Allow use of Gnus search groups as notmuch path: search term"

Fix "Allow use of Gnus search groups as notmuch path: search term"

* lisp/gnus/nnir.el (nnir-notmuch-filter-group-names-function):
  Default to nil -- getting correct behavior requires user
  intervention too often to have this enabled by default.
* lisp/gnus/nnir.el (nnir-run-notmuch): If the user has turned this
  on, then also hardcode `gnus-group-short-name' as a filter -- things
  will never work without it. Also move leading space to before the
  opening parenthesis.
* doc/misc/gnus.texi: Document option.

(Bug#33122)