Profile clarification regarding channels

[cconcolato]

The definition of the Base profile says:

• They SHALL be able to handle up to 18 channels.
  • The 18 channels limit applies to the sum of channels across all Audio Elements in a Mix Presentation before mixing.
  • One example is a mix with 3rd-order Ambisonics (16 channels) + non-diegetic stereo (2 channels).

while the definition of the simple profile says:

• They SHALL be able to handle up to 16 channels.

The latter misses the clarification that the number of channels is the sum.
I suggest moving that clarification in a common section.