return-by or returns-by – consistency needed

@jskeet [noticed](https://github.com/dotnet/csharpstandard/pull/1462#discussion_r3951879638) that we use *return-by-value*, *return-by-ref* & *return-no-value* and versions where it is *returns*. Quick search shows 13 singular and 42 plural occurrences.

The singular form matches the *call-by-X*, or *pass-by-X*, common terms, though they do not occur in the Standard.

Neither singular or plural is *wrong*, but we should be consistent!