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.
Add function in openapi.py to access endpoint descriptions written
in zulip.yaml. Use this function for creating a markdown extension
for rendering endpoint descriptions written in zulip.yaml.
We use this extension for a single endpoint to get test coverage.
This refactors add_reaction in python_examples.py to use the
openapi_test_function decorator and validate result with
validate_against_openapi_schema. Minor changes have been made to the
OpenAPI format data for /messages/{message_id}/reactions endpoint.
This also adds add-emoji.md to templates/zerver/api and adds
add-emoji to rest-endpoints.md (templates/zerver/help/include).