docs/realtime: Add link for understanding long-polling.
docs/realtime: Add link for understanding long-polling.

This is a Wikipedia link which I referred to because I didn't
exactly understand what long polling meant (making the rest of the
doc very hard to understand in turn).

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile