* shortdoc.el (shortdoc-heading): Define new face for headings.
(shortdoc-display-group): Apply new heading face.
(shortdoc--display-function): Use existing face for section text.
* etc/NEWS: Document new face (bug#46748).
* lisp/emacs-lisp/shortdoc.el (shortdoc--display-function): Use
describe-function as a fallback link when a function is not documented
in any Info manual. Also make the link respond to mouse-1, like the
rest of *Help* links, and add a proper help-echo property.
* lisp/help-fns.el (help-fns--mention-shortdoc-groups): Same link
improvement as described before, this time for the shortdoc groups
(bug#45750).
* doc/lispref/sequences.texi (Sequence Functions): Document them.
* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns): Mark them as
side-effect-free.
* lisp/emacs-lisp/shortdoc.el (list): Mention them.
* src/fns.c (Flength): Mention them in the doc string.
(length_internal): New function.
(Flength_less, Flength_greater, Flength_equal): New defuns.
(syms_of_fns): Sym them.
* lisp/emacs-lisp/shortdoc.el (sequence): use seq-contains-p instead
of seq-contains, which is obsolete as of 27.1. (Bug#44536)
Copyright-paperwork-exempt: yes
* lisp/emacs-lisp/shortdoc.el (text-property-search): Require.
(shortdoc-mode): New major mode.
(shortdoc-mode-map): New variable.
(shortdoc--goto-section): New macro.
(shortdoc-next, shortdoc-previous, shortdoc-next-section)
(shortdoc-previous-section): New commands.
(shortdoc-display-group): Use new shortdoc-models. Propertize
section header.
(shortdoc--display-function): Propertize function header.
* lisp/emacs-lisp/shortdoc.el (shortdoc-example): Removed.
(shortdoc-section): Remove colors.
(shortdoc-separator): New face.
(shortdoc-display-group, shortdoc--display-function): Don't use
background colours, because that makes things harder to read.
Separate with a horizontal line instead.
* doc/lispref/help.texi (Documentation Groups): Document it.
* lisp/help-fns.el (help-fns--mention-shortdoc-groups): Output
references to the shortdocs.
* lisp/emacs-lisp/shortdoc.el: New file.