vim: Tweak how comment nesting works

Don't try to match line comments inside of a comment block. That makes
no sense and can highlight differently for people who override their
highlights.

Similarly, don't match a doc-comment inside of a comment block. It
shouldn't be highlighted differently unless it's actually a doc-comment
(and nested comments are obviously not doc comments).

Fixes #12307.
Merge pull request #12308 from kballard/vim-nested-comments

vim: Tweak how comment nesting works

Reviewed-by: brson