Suggesting to add a section of QUIC fec usages

[rachelhuang]

There're 2 usages of QUIC FEC. One is for flows of RTC applications which is this draft mainly for; another, which in our case, is to protect short messages in some scenarios where BDP is large and applications are delay sensitive, e.g., instant messaging, control channels for multimedia conferencing.
In the latter case, short message FEC mechanism uses FEC to produce multiple FEC packets together sent with original packets instead of sending multiple duplicate packets. It would be useful to include texts to prove the current approach can suppport both scenarios. We can help to add some texts if necessary:)