A users own invite event should not be stripped in sync

**Link to problem area**: https://spec.matrix.org/latest/client-server-api/#get_matrixclientv3sync

**Issue**

The spec states that all invite state events should be stripped, but synapse returns the users own invite event in `invite_state` without stripping it. Some clients rely on this to report invite timestamps (based on `origin_server_timestamp`, which is missing in the stripped event).

I asked about this in `#matrix-dev:matrix.org` and @tulir told me:

> oh right, I think the invite event itself is also not stripped, but that detail is missing from the C-S spec
> in the S-S and AS APIs it's more obvious that the invite event isn't stripped, because the stripped state is nested inside the invite event in the unsigned section

**Expected behaviour**

The spec should state that the users own invite event is not stripped.

Start with the linked Matrix Client-Server API sync section and compare its wording with the reported behavior. Update the specification to state that a user's own invite event is not stripped; done means the exception is explicit and consistent with the described invite_state behavior.

Written by the indexing model from the issue text.