Let C-h f do autoloading, and report if a function was previously autoloaded.

Let C-h f do autoloading, and report if a function was previously autoloaded.

* lisp/help-fns.el (help-fns--autoloaded-p): New function.
(describe-function-1): Refer to a function as "autoloaded" if it
was autoloaded at any time in the past.  Perform autoloading if
help-enable-auto-load is non-nil.

* lisp/help.el (help-enable-auto-load): New variable.

* src/doc.c (Fsubstitute_command_keys): Fix punctuation.