mediawiki.cookie: Remove false documentation for options.expiry as number

mediawiki.cookie: Remove false documentation for options.expiry as number

Follows-up 7c585b5. There was never an implementation for
supporting an offset from now in seconds. The code and tests
account for no such thing.

When passed a number, it will actually end up being passed to
$.cookie, which interprets it as an offset in days (!).

Change-Id: I5d8ed8cb432ca550c62cf415077ffffd91fe2bbd