(docs): add missing info about ShareLocks

Hello

I had a production incident a few weeks ago while using deferred indexes, where the Postgres docs lead me down the wrong path of investigation due to missing details. Specifically, the docs implied that a `ShareLock` was only acquired when creating indexes, but only after looking at the code did I learn that this lock is also acquired when transactions are waiting for other transactions to complete. I think this would be helpful to someone who might find themselves on the same path in the future, and as I understand it this mailing list is the way to submit patches to the docs?