Give , and .@ doc strings. Fixes bug #24561.

Give , and .@ doc strings.  Fixes bug #24561.

Also make *Help* links to ``' possible.  Also make usable as such doc strings
on the function-documentation property of a symbol.

* lisp/emacs-lisp/backquote.el (top-level): Give , and '@ doc strings on the
function-documentation property.  Also give these symbols a reader-construct
property.

* lisp/help-fns.el (describe-function): Allow the function-documentation
property to work.  Use princ rather than prin1 to print the function's name
when it has a reader-construct property.
(help-fns-signature): Don't insert `high-usage' for a reader-construct.
(describe-function-1): Adapt to process documentation on the
function-documentation property.  Print "a reader construct" when appropriate.

* lisp/help-mode.el (help-xref-symbol-regexp): Amend this regexp also to match
``'.