bug#64902: 30.0.50; REQUIRE-MATCH completing-read arg in describe-* commands

[Stefan Monnier]

> The core of the problem is that in the normal UI, `confirm` is very
> lightweight (you'll basically only notice it when you do try to enter
> the name of a function that's not defined) whereas in the Helm UI it's
> harder to hide it.

In the long run, maybe the better option would be to move the `confirm`
info from `require-match` to `test-completion`: make it so
`test-completion` can return a value which says "this may be a valid
completion but it requires confirmation from the user".

This way `describe-function` could distinguish between names which are
sure to correspond to nothing (e.g. there is no matching symbol in
`obarray`, or there is but it has both `symbol-function` and
`symbol-plist` are empty) and names where `symbol-function` is nil, but
there are some properties defined.


        Stefan