openapi: Add comments on top of various OpenAPI doc files.

This is part of our standard approach for trying to make it easy for
folks to find relevant documentation on a system they're trying to
understand.

zerver/openapi/test_curl_examples.py

@@ -1,3 +1,10 @@
+# Zulip's OpenAPI-based API documentation system is documented at
+#   https://zulip.readthedocs.io/en/latest/documentation/api.html
+#
+# This file contains the top-level logic for testing the cURL examples
+# in Zulip's API documentation; the details are in
+# zerver.openapi.curl_param_value_generators.
+
 import glob
 import html
 import json