gpu: improve documentation related to i915 resource and align with GAS' documentation

[tkatila]

In #1377 there has been confusion about what a i915 resource actually means in the Pod spec. Improve the documentation so that the definition of i915 resource is clear in all the use cases (shared-dev-num=1, shared-dev-num>1 and GAS).

[eero-t]

Isn't this dup of your comment in #1401 ?