Since we already have three of these, and I'm about to add a fourth, let's pull
it out to a common definition.
We could, of course, keep defining the grammar each time it's used, but
defining it in an appendix helps us be consistent for future API design.
* "MXC URI" -> "`mxc://` URI"
We're a bit inconsistent with this currently, and IMHO "`mxc://` URI" is more
explicit.
* Update content/client-server-api/modules/content_repo.md
Co-authored-by: Hubert Chathi <hubertc@matrix.org>
* more MXCs
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Co-authored-by: Hubert Chathi <hubertc@matrix.org>
