Clarify that lazy_load_members in timeline isn't respected in /sync

[bradtgmurray]

This one caught me out today, clarify that sync only uses lazy_load_members from state.

https://github.com/element-hq/synapse/blob/develop/synapse/api/filtering.py#L240-L241

Pull Request Checklist

• [ ] Pull request includes a changelog file
• [x] Pull request includes a sign off
• [x] Pull request is classified as 'other changes'

Signed-off-by: Brad Murray brad@beeper.com

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[turt2live]

Are we sure this isn't a synapse bug?