Add max_frame_size option for websocket handlers

Option allows to limit a frame by size before decoding its payload.

LH: I have added a test for when the limit is reached on a nofin
fragmented frame (the last commit addressed that case but it had
no test). I have fixed formatting and other, and changed the
default value to infinity since it might otherwise be incompatible
with existing code. I also added documentation and a bunch of other
minor changes.

doc/src/guide/ws_handlers.asciidoc

@@ -225,6 +225,22 @@ init(Req, State) ->
 This value cannot be changed once it is set. It defaults to
 `60000`.
 
+=== Limiting frame sizes
+
+Cowboy accepts frames of any size by default. You should
+limit the size depending on what your handler may handle.
+You can do this via the `init/2` callback:
+
+[source,erlang]
+----
+init(Req, State) ->
+    {cowboy_websocket, Req, State, #{
+        max_frame_size => 8000000}}.
+----
+
+The lack of limit is historical. A future version of
+Cowboy will have a more reasonable default.
+
 === Saving memory
 
 The Websocket connection process can be set to hibernate