Add FAQ entry about E2EE of calls in FA

(originally asked on the vector.im repo before it was archived)

I was surprised not to find on the Web whether Element calls were E2EE until I asked about it on #riot-web. I suggest adding an entry such as the following to the [end-to-end encryption section of the Element FAQ](https://element.io/help#end-to-end-encryption), assuming it is correct:

**Are Element calls end-to-end encrypted?**

Audio and video calls are end-to-end encrypted in one-to-one chats with end-to-end encryption enabled (the number of devices doesn't matter).

End-to-end encryption of calls is not supported in group rooms.

See also: Discussion in [matrix.org issues](https://github.com/matrix-org/matrix.org/issues/614).