* lisp/help-fns.el (describe-command): New command.
(help-fns--describe-function-or-command-prompt): New helper
function to prompt for a function or function. (Bug#46627)
(describe-function): Use above new helper function.
* lisp/help.el (help-map): Bind above new command to `C-h x'.
(help-for-help): Add this new command to the help summary.
* lisp/menu-bar.el (menu-bar-describe-menu): Add the new command to
the help menu.
* doc/emacs/help.texi (Help Summary, Name Help): Document
'describe-command', and update documentation on 'describe-function'.
* etc/tutorials/TUTORIAL: Change reference from 'describe-function' to
'describe-command'.