Better document what erase: true when deactivating a user does

[babolivier]

Currently the doc (or lack thereof, at least for the non-admin side of things) can lead people to think what's when setting erasure to true upon deactivation is that we wipe the deactivated user's messages from the server.

What's actually happening is that the messages don't actually get deleted from the database (because it's a tricky thing to do), and people who were in the room when the messages were sent can still see them (the same way when someone deletes their email account the emails they've sent are still present in the recipients' mailboxes), but they will be hidden to new people joining the room after the messages were sent.

We should make that clearer. This could be updating https://github.com/matrix-org/synapse/blob/develop/docs/admin_api/user_admin_api.rst#deactivate-account so that it mentions that, or create a GDPR doc that explains it (or find a better fit place to document this).

[babolivier]

Might also be worth mentioning there that erasure requests aren't federated till matrix-org/matrix-spec-proposals#2438 is fully supported.