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

[Drew Adams]

> > > The main difference is in the completion these commands
> > > offer.  describe-command completes only on commands.
> >
> > Yes, that's an important difference.  In addition,
> > _only commands are accepted_ as input, regardless
> > of whether you use completion.
> 
> No, because RET does completion.

(We're getting into nits now.  OK.)

Only if REQUIRE-MATCH is non-nil.  And _that_
provides the feature I mentioned: only commands
are accepted as input.

With REQUIRE-MATCH = nil, RET does no completion:

(completing-read "Command?: " obarray 'commandp nil)

You can input `xxxxxxxxxxxxxxxxxxxx' if you like,
and it's accepted without being a command name.

What you emphasized is the result of a PREDICATE
arg of `commandp'.  But accepting only commands
has nothing to do with completing.  It has to do
with REQUIRE-MATCH.  Both are important here.