Clarify how to handle key sharing wrt history visibility for invites

[HarHarLinks]

Link to problem area:
https://spec.matrix.org/latest/client-server-api/#end-to-end-encryption

Issue
While history key sharing is now specified for history visibility including changes between "shareable" and not invite/join visibilities, the spec is missing clarity that when a user is invited to a room, their devices SHOULD(?) be considered a member of the megolm group from that point onward.

Element introduced this behavior in matrix-org/matrix-js-sdk#666.

Having discussed with @richvdh, I will propose a clarification PR for this.

[HarHarLinks]

So the difficulty here is to find the right place, because the equivalent description for the behavior in case of join membership when history visibility is join is also not defined as far as I can find. In fact the whole behavior of "what happens when encrypted" is basically not defined on that level, or I can't find it so far.