TT#14008 clarify codec options for `publish`

reported in #1538

Change-Id: Ib38ebca71a67f727bec4f545700e87ce22230c56
(cherry picked from commit b8ccbcc2ae)

README.md

@@ -1390,6 +1390,9 @@ Optionally included keys are:
 		torwards the original offerer and then used for transcoding. It is a more selective
 		version of what the `always transcode` flag does.
 
+		The special string `any` can be used for the `publish` message.
+		See below for more details.
+
 	* `set`
 
 		Contains a list of strings. This list makes it possible to set codec options
@@ -2214,7 +2217,17 @@ this is not a rewritten version of the received SDP, but rather a `recvonly`
 answer SDP generated by *rtpengine* locally. Only one codec for each media
 section will be listed, and by default this will be the first supported codec
 from the published media. This can be influenced with the `codec` options
-described above.
+described above, in particular the `accept` option.
+
+The list of codecs given in the `accept` option is treated as a list of codec
+preferences, with the first codec listed being the most preferred codec to be
+accepted, and so on. It is allowable to list codecs that are not supported for
+transcoding. If no codecs from the `accept` list are present in the offer, then
+the first codec that is supported for transcoding is selected. If no such codec
+is present, then the offer is rejected. The special string `any` can be given
+in the `accept` list to influence this behaviour: If `any` is listed, then the
+first codec from the offer is accepted even if it's not supported for
+transcoding.
 
 `subscribe request` Message
 ---------------------------