Add FAQ to the docs

docs/faq.rst

@@ -0,0 +1,28 @@
+**************************
+Frequently Asked Questions
+**************************
+
+API/Design
+==========
+
+*Why have a separate engine for server and client?  Surely traffic
+could be differentiated as much as it needs to be internally in one
+engine?*
+
+The traffic *cannot* be differentiated for gQUIC versions Q046 and Q050.
+This is because in these versions, the server never includes a connection
+ID into the packets it sends to the client.  To have more than one
+connection, then, the client must open a socket per connection: otherwise,
+the engine would not be able to dispatch incoming packets to correct
+connections.
+
+To aid development, there is a :macro:`LSQUIC_FORCED_TCID0_VERSIONS` that
+specifies the list of versions with 0-sized connections.  (If you, for
+example, want to turn them.)
+
+Once gQUIC becomes deprecated in the future, there will remain no technical
+reason why a single engine instance could not be used both for client and
+server connections.  It will be just work.  For example, the single
+engine settings :type:`lsquic_engine_settings` will have to be separated
+into client and server settings, as the two usually do need to have
+separate settings.

docs/index.rst

@@ -66,6 +66,7 @@ Contents
    tutorial
    apiref
    internals
+   faq
 
 Indices and tables
 ==================