api-docs: Add warning formatting to important description notes.
api-docs: Add warning formatting to important description notes.

Adds warning block formatting around important notes in the
`/get-events` and `/dev-fetch-api-key` endpoint documentation.

rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,rht/zulip