Document how monthly active users is calculated

[Half-Shot]

We document the individual configuration parameters in https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html?highlight=monthly#limit_usage_by_mau, but it would be good to have a separate document stating how the system works at a high level.

In particular, I'd like to see:

• How activity is calculated.
• How the "trial period" works.
• How users (including deactivated) users may fall out of that activity period.
• When the user limit is reached, what services/endpoints are blocked by the homeserver.

It would be good to have this from the point of view of having something to point at when someone asks how it works.

[richvdh]

+1. MAU has always been something of a mystery to me.