bug#30206: 27.0.50; Minibuffer completion should not complete internal names

[Drew Adams]

> C-h f abbrev TAB
> 
> You'll get completions for internal functions such as abbrev--describe.

Of course.  As well you should.

> By default minibuffer completions shouldn't complete such internal
> names, as users aren't supposed to use them directly.

This is ridiculous, IMHO.  Users are not supposed to use
"internal" functions directly?  Nonsense.

Besides which, "use" is meaningless here.  Completing
a name doesn't imply calling the name as a function or
evaluating it as a variable.  And there is nothing wrong
with doing either of things, anyway.  Would you also
forbid `mapatoms' from using (acting on) "internal" names?

"Minibuffer completions" means multiple things, anyway.

Anyone can use `completing-read' or any other minibuffer
completion function in a way that does not complete
"internal" names, if they want to.