Improve SSL explanation in the StreamPeerSSL documentation

**Your Godot version:** 3.2.3

**Issue description:**

The StreamPeerSSL description doesn't explain much what SSL is used for, what guarantees it gives and where the certificates can be loaded from. All this should be described somewhere in the class.

See See https://wolthera.info/2020/12/xmpp-client-in-godot/.

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/latest/classes/class_streampeerssl.html