This PR updates the tenses used by the summary line of doc comments to
match the [Rust API documentation
conventions](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#summary-sentence).
Specifically:
> The summary line should be written in third person singular present
indicative form. Basically, this means write ‘Returns’ instead of
‘Return’.
I'm sure there are plenty occurrences that I missed.
Release Notes:
- N/A
01f06f96a1