bug#46627: [PATCH] Add new help command 'describe-command'

[Howard Melman]

FWIW since I started using a third party completion package
(first helm, then ivy and now selectrum/consult/marginalia)
I use apropos far less.  I didn't expect that but it's the
case for me.

I've configured these package so that they show possible
completions vertically, one per line, and show annotations.
So now when I type M-x I not only see a list of commands I
can invoke, but also their keybinding and the first line of
their docstring.  As I type and narrow the completions this
shows me a list of commands related to what I want and even
a brief description if the name is not enough.  

I know that apropos is more featureful and I know how to
invoke it easily, but since I use M-x more commonly and it
now includes this basic info, my use of C-h a has decreased
as its just anothing thing to remember.

Also, when I use M-x normally I now get a kind of background
appropos happening all the time which gives me a little more
discovery.  If I search for "register" to find the
jump-to-register command I'll see other register commands,
and those that are bound stand out a bit. If I'm not in a
rush I might scan it to discover other things I didn't know
about.

This works for me to with C-h v whose annotations show me
the current value (in a clipped narrow column) and the first
line of the docstring.  Also with C-h f.  These are not as
complete as apropos as they don't match on the docstrings,
but they're more interactive and I've found are good enough
and apropos is still there if I need it.

-- 

Howard