Improve RTPCodecParameters docs (#178)

lib/ex_webrtc/rtp_codec_parameters.ex

@@ -5,6 +5,18 @@ defmodule ExWebRTC.RTPCodecParameters do
 
   alias ExSDP.Attribute.{FMTP, RTPMapping, RTCPFeedback}
 
+  @typedoc """
+  RTP codec parameters.
+
+  * `payload_type` - payload type used to identify the codec.
+  Keep in mind that the actual payload type depends on who sends the SDP offer first.
+  If the browser sends it first and uses a different payload type for the same codec, 
+  Elixir WebRTC will override its settings and use the payload type provided by the browser.
+  If Elixir WebRTC sends the offer first and uses a different payload type for the same codec, 
+  the browser will override its settings and use the payload type provided by Elixir WebRTC.
+
+  For the meanings of the other fields, refer to the [MDN documentation](https://developer.mozilla.org/en-US/docs/Web/API/RTCRtpSender/getParameters#codecs)
+  """
   @type t() :: %__MODULE__{
           payload_type: non_neg_integer(),
           mime_type: binary(),