* Consistent decryption tail for all AEADs

* Remove outdated note

This was previously copied here from another function. There used
to be another comment on the tag verification linking to issue #1776,
but that one was not copied over. As it stands, this note seems fairly
misleading/irrelevant.

* Prettier docs

* Add note about plaintext contents to docs

* Capitalization

* Fixup missing XChaChaPoly docs