(imenu--generic-function): Doc fix. Rewritten to be faster.

(imenu--generic-function): Doc fix.  Rewritten to be faster.
(defgroup imenu): Add :link.
(imenu-use-markers, imenu-auto-rescan-maxout, imenu-generic-expression)
(imenu--make-index-alist, imenu-default-goto-function): Doc fixes.
(imenu-max-item-length, imenu-sort-function)
(imenu-scanning-message): Custom tweak.
(imenu-progress-message): Use real backquote syntax.
(imenu--in-alist): Unused function deleted.
(imenu--flatten-index-alist): Likewise.
(imenu-case-fold-search): Add autoload cookie.
(imenu--completion-buffer): Offer function at point as default.
(imenu--subalist-p): Don't use caadr.
(imenu): Don't use caddr.
(imenu-add-menubar-index): New function.