runtime(doc): add more pointers to 'completeopt'

runtime(doc): add more pointers to 'completeopt'

Before this commit, I had trouble finding information about configuring
the insert mode completion. In particular, it was not clear that the
'wildopt' config that I already had in my vimrc does not apply here.

Also, `insert.txt` barely mentioned 'completeopt' except when
describing popups (I was more interested in bash-like behavior
where the unique prefix of all completions is completed first).

I'm hoping these edits will make the relevant docs easier to find.

closes: #17515

Signed-off-by: Ilya Grigoriev <ilyagr@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>