Add a note in docs to check isInWriteTransaction before write transactions in notification blocks

[pigeondotdev]

There was a discussion about checking realm.isInWriteTransaction before making a write transaction while in a notification block. See #4511 for more information. Created this issue to track the progress on possibly adding a note to the documentation about this.

@evgeny-sureev: It must be stated in documentation that notifications may come inside write transaction and it is user's responsibility to check realm.isInWriteTransaction before attempting write.

[jpsim]

The more important point to document here is that notifications may be fired synchronously when advancing by calling -beginWrite, at which point the notification block will be run during that write transaction.

Recommending to check -[RLMRealm isInWriteTransaction] before making a write transaction while in a notification block might be the right solution in some cases, but documenting the cases in which that can happen will give the user more insight into what's actually happening.

[jpsim]

I've unassigned myself since I don't think I'll be able to get to this any time soon.