Refine and clarify how presence works

[ara4n]

Documentation: https://docs.google.com/document/d/1sKaM9J5oorEeReYwOBmcgED6XnX2PdCYcx0Pp0gFnqM/edit#

Edited by TravisR to make this a proposal - original body below

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The presence API is better than it used to be, but still very confusing in some respects. Custom presence is particularly broken, as @turt2live discovered in matrix-org/matrix-react-sdk#1676.

I've made a proposal at https://docs.google.com/document/d/1sKaM9J5oorEeReYwOBmcgED6XnX2PdCYcx0Pp0gFnqM/edit# - if someone can review it then I'll do a spec PR (and a synapse PR) to implement it. It will be largely backwards compatible with the current behaviour.

[turt2live]

@ara4n I know holidays, vacation, and ops have gotten in the way recently, but has there been any traction on this front?