Clarify how many pushers can be associated with a given access token

**Link to problem area**:

https://spec.matrix.org/latest/client-server-api/#push-notifications

**Issue**

The definition of a pusher in that section is fairly confusing and also a bit misleading by saying there can only be "one per device". It is unclear what "device" is referring to here (since Matrix itself has a concept called a "device"). If we're talking about Matrix devices, the restriction that one device can only have one pusher is quite impractical, since it means clients cannot register both an HTTP pusher and an email one. It is also worth noting that Synapse does not enforce any restriction on the number of pushers associated with a given device (and never has as far as I can tell).

Start with the Push Notifications section linked in the issue, especially the definition of a pusher and its statement about one pusher per device. Review how the surrounding specification describes access tokens and Matrix devices. The work is done when the specification clearly explains what “device” means and how many pushers may be associated with an access token or device.

Written by the indexing model from the issue text.