12.1 Update table instructions to indicate explicit role precedence

[kengdoj]

Table data cell: Check that each data cell uses only one of the following methods

<table role="table"> must use aria cell, row, header roles in steps 3 and 4.

[kengdoj]

For reference: https://www.w3.org/TR/html-aria/#docconformance

[kengdoj]

Explicit role=table requires the supporting roles for row, header, etc.

<table role="table"> is not a failure, but because an explicit role for table has been defined, child table elements must have explicit roles defined.

[kengdoj]

also add that a cell's assigned header ID values must be within the same table as the cell