Warn methods in doc comments written as `fun()` instead of `fun`

[clarfonthey]

According to #41771 (comment), the current convention for rustdoc is to write methods without any trailing parentheses. For example, the comment "Resizes the Vec in-place so that len() is equal to new_len" should really be "Resizes the Vec in-place so that len is equal to new_len".

Would be nice if we had some way for clippy to warn about this convention.

[clarfonthey]

Note: the original comment on this disputed the usefulness of this convention, so, before this is implemented there should be a consensus on whether the parentheses are preferred or if we'd rather not have them.