Other details about completion.

[Ergus]

Hi all:

After some days using the new completion customs I have found some other
details that maybe could be `improved'.

1) When display-line-numbers is enabled the extra lines (help and
header) produce a mismatch between the line number and the current
candidate.

I know we can live with it... But... maybe there is a way to add a
padding or a property or anything that inhibits the display engine from
producing numbers for some lines or a line range... even if we restrict
it to only the beginning lines and internal use? I know there is some
code to do that with continuation lines...

2) Do we have a property or any way to make the commands in M-x to show
some help? Like the one shown when `completions-detailed` is non-nil in
`C-h f`?

Best,
Ergus