[Docs] Update GEP docs for opaque pointers

Update the GEP FAQ to use opaque pointers. This requires more than
a syntactic change in some place, because some of the concerns just
don't make sense anymore (trying to index past a ptr member in a
struct for example).

This also fixes uses of incorrect syntax to declare or reference
globals.

Differential Revision: https://reviews.llvm.org/D130353