MSC2530: Body field as media caption (#2530)
* Proposal to use body field as media caption
* Add paragraph about relation-based captions being difficult for bridges
* Clarify how to treat body when filename is not present
* Refactor proposal text
* Fix heading size
* Add problem statement
* Add links to and quotes from current spec
* Adjust wording and quote m.audio body spec
* Clarify that m.location and m.sticker are out of scope for this proposal
* Add examples and summary of changes
* Fix JSON syntax in example
