Support newer javadoc features

[owengray-google]

Describe the bug
As of java 16, the javadoc spec now supports an inline {@return <description} tag. This tag is used at the beginning of a description, and both renders as "Returns <description>" in primary documentation text, and also creates the Returns tag as though it also were a normal @returns <description>.
This inline form is used in some of our Google non-androidx clients, as only android is restricted to a low Java version.
It isn't understood by dokka, which renders it as a text literal.

It is likely that there may be other updates to the javadoc spec in versions of Java that Kotlin and/or Android aren't very interested in. The internal documentation page that describes inline return and when it was added does not mention any other new javadoc spec, but I don't expect it to be comprehensive. I'm not sure what the best way to find newly-added javadoc spec would be.

Expected behaviour
Inline {@return } javadoc tags work.
(As do any other recently-added javadoc features)