Reduce doc duplication in general and for version-related params

Group `as_of`, `issues`, and `lag` params together under "@param as_of, issues, lag. [versioning] params, mutually exclusive" and link to a separate page that goes in-depth about these parameters. Will help remove a bunch of doc duplication.

More generally, reduce doc duplication/maintenance by using roxygen templates to keep param text in one place only.