Improve clarity of channel ordering

[felicialim]

A couple of places where clarity could be improved:

1. Clarify that the channel ordering in the bitstream is separate from the channel order of the encoder's input or the decoder's output. The former is specified in 3.6.3.3. Ordering of Audio Substream Identifers and the latter is implementation dependent.

2. The list of channels provided in https://aomediacodec.github.io/iamf/#coupled_substream_count could be reordered to follow 3.6.3.3. Ordering of Audio Substream Identifers.

[sunghee-hwang]

I am still thinking about the interface between IAMF decoders and Renderers. We may need to specify the channel order of the decoder's output.

[jwcullen]

I'm thinking we could clean up a lot of language by providing an canonical list of channels, with long names, ordered by assignment precedence in the bitstream.

Something like this:

Coupled substreams

Short Names	Long Names
L / R	Left / Right
Ls/Rs	Left side / Right Side
Lss/Rss	...
...	...

Uncoupled substreams

Short Name	Long Name
C	Centre
FC	Front Centre
LFE	...
LFE 1	...
LFE 2	...
L	Left (for mid-side)

I think if written nicely, this could replace several sections:

1. Explanation of which substreams are coupled.

Each pair of coupled stereo channels in the same Channel Group SHALL be coded in stereo mode [...]. Coupled stereo channels: L/R, Ls/Rs, Lss/Rss, Lrs/Rrs, Ltf/Rtf, Ltb/Rtb, [...] Non-coupled channels: C, LFE, L, FC, LFE1

2. Dense list of of "long names".

Where FLc: Front Left Centre, FC: Front Centre, FRc: Front Right Centre, FL: Front Left, [...]

3. The plain language section establish channel assignment of substreams is verbose and difficult to read the intended order.

The order of the Audio Substreams in each Channel Group [...] Coupled substreams come first and are followed by non-coupled substreams. [...]

[sunghee-hwang]

@jwcullen thank you for the suggestion. Let me consider it too.