Clarify the definition of prerequisites to commands

[kpet]

• Reword the first source of prerequisites so the wording is symmetrical with respect to the others (i.e. the first/second/third ...).
• Broaden the first source of prerequisites to cover all implicit dependencies and provide an exhaustive list of how they arise: either because of barriers or because of ordering in in-order command-queues.

Change-Id: Ic464066261fe13756347bafb4878cd6ffb5a8427