"InviteState" in sync response has inconsistent contents

This issue has been migrated from [#4363](https://github.com/matrix-org/synapse/issues/4363).

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### Description

The /sync client endpoint specifies the `InviteState` as a list of `StrippedState` and reads about these events that

> These state events may only have the `sender`, `type`, `state_key` and content keys present.

I see two ways to interpret this sentence (I find the "may only" ambiguous):

- either this means that these events are only allowed to contain these keys, in which case synapse is inconsistent with the spec in that it also sends non-stripped events in this list
- either this means that these events are allowed to only have these fields, in which case it is contradictory with the later definition that the contents of `InviteState` is `[StrippedState]`

### Steps to reproduce

assuming the first interpretation, #4359 contains an example JSON returned by synapse containing a mix of stripped and non-stripped events

### Version information

- **Homeserver**: my personal HS

If not matrix.org:

- **Version**: 0.34.0

- **Install method**: pip

- **Platform**: Gentoo/Linux in virtualenv