Sliding Sync: Add comment to explain extra case where you can be invited ->...

Sliding Sync: Add comment to explain extra case where you can be invited -> banned -> unbanned (#17654)

Add comment to explain extra case where you can be
invited -> banned -> unbanned and we want to be able
to find the invite event.

Follow-up to https://github.com/element-hq/synapse/pull/17636#discussion_r1738993330