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.
Currently response return values have to be written twice, once in
the docs and once in zulip.yaml. Create a markdown extension so
that the return values in api docs are rendered using content from
zulip.yaml
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.