Clean up comments to be careful about the distinction between variable-

Clean up comments to be careful about the distinction between variable-
width types and varlena types, since with the introduction of CSTRING as
a more-or-less-real type, these concepts aren't identical.  I've tried to
use varlena consistently to denote datatypes with typlen = -1, ie, they
have a length word and are potentially TOASTable; while the term variable
width covers both varlena and cstring (and, perhaps, someday other types
with other rules for computing the actual width).  No code changes in this
commit except for renaming a couple macros.