api: Complete get_events response format documetation.
api: Complete get_events response format documetation.

This giant commit completes basic OpenAPI documentation for all events
in Zulip's real-time events API.

Further work will be required in the near future to make
/api/get_events usable.

With many edits by tabbott for wording and correctness (especially
around which clients receive events, and their purpose).

hackerkid/zulip,kou/zulip,hackerkid/zulip,brainwane/zulip,punchagan/zulip,showell/zulip,showell/zulip,kou/zulip,andersk/zulip,zulip/zulip,brainwane/zulip,zulip/zulip,kou/zulip,rht/zulip,brainwane/zulip,rht/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,showell/zulip,brainwane/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,punchagan/zulip,eeshangarg/zulip,zulip/zulip,showell/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,showell/zulip,kou/zulip,kou/zulip,kou/zulip,rht/zulip,rht/zulip,zulip/zulip,kou/zulip,brainwane/zulip,brainwane/zulip,andersk/zulip,punchagan/zulip,rht/zulip,andersk/zulip,brainwane/zulip,rht/zulip,punchagan/zulip,showell/zulip,eeshangarg/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,showell/zulip,hackerkid/zulip,andersk/zulip