openapi: Use "description: |" for multiline paragraphs.

"description: |" supports markdown and is overall better for
writing multiline paragraphs. So use it in multiline paragraphs
and line-wrap the newly formed paragraphs accordingly.

Edited by tabbott to change most single-line descriptions to use this
format as well.

zerver/tests/test_openapi.py

@@ -57,7 +57,7 @@ class OpenAPIToolsTest(ZulipTestCase):
             'name': 'message_id',
             'in': 'path',
             'description':
-                'The ID of the message that you wish to edit/update.',
+                'The ID of the message that you wish to edit/update.\n',
             'example': 42,
             'required': True,
             'schema': {'type': 'integer'}