Add new help command 'describe-command'

* 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'.
6 jobs for master in 9 minutes and 57 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Fast
failed #18641
test-fast-inotify

00:06:52

 
  Normal
failed #18642
test-lisp-inotify

00:03:05

passed #18643
test-lisp-net-inotify

00:02:32

 
  Slow
skipped #18644
build-native-bootstrap-speed0
skipped #18645
build-native-bootstrap-speed1
skipped #18646
build-native-bootstrap-speed2
 
Name Stage Failure
failed
test-lisp-inotify Normal
Makefile:257: recipe for target 'check-lisp' failed
make: Leaving directory '/checkout/test'
Running after_script
Running after script...
$ test -n "$(docker ps -aq -f name=${test_name})" && docker cp ${test_name}:checkout/test ${test_name}
$ test -n "$(docker ps -aq -f name=${test_name})" && docker rm ${test_name}
test-lisp-inotify-6c1c3204
Cleaning up file based variables
ERROR: Job failed: exit code 2
failed
test-fast-inotify Fast
make[1]: *** [check-doit] Error 1
make: *** [check] Error 2
Running after_script
Running after script...
$ test -n "$(docker ps -aq -f name=${test_name})" && docker cp ${test_name}:checkout/test ${test_name}
$ test -n "$(docker ps -aq -f name=${test_name})" && docker rm ${test_name}
test-fast-inotify-6c1c3204
Cleaning up file based variables
ERROR: Job failed: exit code 2