Docs, OpenMP: Correct internal documentation of OMP_FOR.

Docs, OpenMP: Correct internal documentation of OMP_FOR.

gcc/ChangeLog:

	* doc/generic.texi (OpenMP): Document OMP_SIMD, OMP_DISTRIBUTE,
	OMP_TASKLOOP, and OMP_LOOP with OMP_FOR.  Document how collapsed
	loops are represented and which fields are vectors.  Add
	documentation for OMP_FOR_PRE_BODY field.  Document internal
	form of non-rectangular loops and OMP_FOR_NON_RECTANGULAR.

	* tree.def (OMP_FOR): Make documentation consistent with the
	Texinfo manual, to fill some gaps and correct errors.