Add information about which codecs are supported for bi-directional audio with ExternalMedia

[Alge]

The documentation for External Media channels currently include this:

format: (required) The format/codec you wish the media to be encoded in.Any standard format/codec supported by Asterisk is supported here. For example: ulaw, g722, etc.There is no negotiation. The format you specify is the format you'll get. The channel driver will automatically transcode the bridge's native media into this format.
(https://docs.asterisk.org/Development/Reference-Information/Asterisk-Framework-and-API-Examples/External-Media-and-ARI/)

And on this page the codec limitations are not mentioned at all: https://docs.asterisk.org/Asterisk_22_Documentation/API_Documentation/Asterisk_REST_Interface/Channels_REST_API/#externalmedia

This has lead me and others to think that all codecs supported by asterisk can be used with the externalMedia channel, but this is not the case, only statically defined payloads are:

(https://community.asterisk.org/t/dont-hear-audio-streamed-back-to-external-media-channel/94897/2)

I think it would save a lot of people quite a lot of debugging if we could include more clear information about which codecs work, as this issue does not trigger any form of error messages or log rows, the only symptom is that audio to the asterisk server is dropped and not played in the call (outbound audio from asterisk does still seem to work with dynamic payload types like OPUS and slin24).

[gtjoseph]

I'm going to be re-writing the external media stuff over the next few weeks so I'll make sure to be more clear.