Mostly, this is a change in ordering to make more sense, but we also
fix several names that were clearly confusing.
We restore the convention that each endpoint has the same title at the
top of the page as what we have in the sidebar menu, which appears to
have been violated in many recent updates to API documentation.
The term `parameter` is a better word than `argument` for data passed
to an API endpoint; this is why OpenAPI uses in their terminology.
Replace `argument` with `parameter` in the API docs to improve their
readability.
Fixes#15435.
Firstly, change endpoint descriptions in zulip.yaml so that they
match their counterpart in the api docs. Then edit the api docs
so that they use api description markdown extension for displaying
endpoint description.