Docs should probably show certain #[verifier(...)] attribute declarations

[jaybosamiya]

This came up initially due to when_used_as_spec (in #430), but likely will apply to other attributes we might want to expose to the user, so opening as a discussion.

For example, Vec::len should show that when .len() is used in spec , it'll use Vec::spec_len due to the #[verifier(when_used_as_spec(spec_len))]

CC: @tjhance