Rate limiting is sparsely mentioned

The client/server API mentions rate limiting in quite a few places, however there's still the vast majority of endpoints not being declared as rate limited.

The endpoints that don't declare rate limiting are: https://github.com/turt2live/matrix-doc/commit/cb1b58d281801fba6fec1b9de492287b689c3da2

Synapse is a little weird in this area. It appears as though there's no global rate limiter (needs verification), and in many places rate limiting is explicitly done. There's also a few places where it's just outright disabled. For reference: https://github.com/matrix-org/synapse/search?q=ratelimit&type=Code

Start with the linked commit cb1b58d281801fba6fec1b9de492287b689c3da2 to identify the API endpoints missing rate-limit declarations, then review the Synapse ratelimit search linked in the issue. Verify which endpoints are actually limited and document the agreed rate-limiting declarations across the specification.

Written by the indexing model from the issue text.