Improve scalable channel group and layer text

[cconcolato]

The current text is spread:

• in the audio_substream_id semantics
• in 3.6.2. Scalable Channel Layout Config Syntax and Semantics, with some text mixed in the Syntax sub-section, before the Semantics sub-section
• in 3.6.2.1. Channel Layout Generation Rule and 3.6.2.2. Channel Group Format

It would be better if all of it was under a "scalable channel group and layout" section.

Also there are some typos in some formulas. For example:
"c = [1, ..., C] is the Channel Group index and
is the number of Channel Groups."
I think = should be ∈. If c is an index, it cannot be equal to an array.
Same for n_c.