Add additional non-uniform/explicit tile details.

[todd-n]

Add explanation for MaxTileHeightSb limits when non-uniform/explicit tiles are used.
Based on discussion from:
https://bugs.chromium.org/p/aomedia/issues/detail?id=2559&q=

[imoccaga]

What is the reason for this restriction, and does this restriction cause existing encoder / bitstreams to become non-legal ?

[peterderivaz]

@imoccaga The change is in a note. Notes do not affect the normative decoding process but try to provide additional context or explanation for the reader.

This note does not add a restriction, but tries to explain the reason for a restriction that is already present in the specification.

Therefore it would not cause existing encoder / bitstreams to become illegal.

Personally, I think the note is helpful and would be happy for it to be included, but it is up to the editors to decide whether it is significant enough to be worth updating.

[imoccaga]

I missed the "Note:", thanks.

I suggest some additional people take a look to confirm that this note is technical correct.