Add entry about E2EE of calls in Riot FAQ

I was surprised not to find on the Web whether Riot 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 Riot FAQ](https://about.riot.im/help#end-to-end-encryption), assuming it is correct:

**Are Riot 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.