stray references to SubscriptRef type

There are several mentions in code comments of a SubscriptRef type, but
that type does not exist and AFAICT never existed. (Perhaps it existed
in in-development code at some point.)

I think in several of those cases this clearly meant the type
SubscriptingRef instead. But the mentions in
src/include/nodes/subscripting.h probably meant the various sbs_* functions.

If so, it would also be useful perhaps to document the leakproof
expectations of the other sbs_* functions (sbs_fetch_old and
sbs_check_subscripts).

See attached patch for my solution so far.