[DOC] - cXML, Stream, new attribute, "codec"

[giavac]

Describe the requested update
cxml Stream (https://developer.signalwire.com/compatibility-api/cxml/voice/stream) now has a new attribute, codec.
It's an optional string, and can have the values L16@24000h or L16@16000h.
This attribute allows specifying a codec different than 8 KHz PCMU if desired.

Reason for the request

New feature available: https://github.com/signalwire/cloud-product/issues/13752

Sections to be updated

https://developer.signalwire.com/compatibility-api/cxml/voice/stream, "attributes"

Additional context

Related Issues or PRs
https://github.com/signalwire/cloud-product/issues/13752