[flang][NFC] Document extension: scalars are trivially contiguous

The Fortran 2018 standard defines the concept of simple contiguity
in subclause 9.5.4 as a characteristic of arrays.  So that scalars
may also be used in contexts where simply contiguous arrays are
allowed, f18 treats them as single-element arrays that are trivially
contiguous.  This patch documents this semantic extension and
also adds comments to the predicate that implements the concept.

Differential Revision: https://reviews.llvm.org/D111679