Clarify handling of special-case values in bootstrap catalog data.

Clarify handling of special-case values in bootstrap catalog data.

I (tgl) originally coded the special case for pg_proc.pronargs as
though it were a kind of default value.  It seems better though to
treat computable columns as an independent concern: this makes the
code clearer, and probably a bit faster too since we needn't do
work inside the per-column loop.

Improve related comments, as well, in the expectation that there
might be more cases like this in future.

John Naylor, some additional comment-hacking by me

Discussion: https://postgr.es/m/CAJVSVGW-D7OobzU=dybVT2JqZAx-4X1yvBJdavBmqQL05Q6CLw@mail.gmail.com