Document the always-true-but-previously-undocumented fact that PQfnumber()

Document the always-true-but-previously-undocumented fact that PQfnumber()
will downcase the supplied field name unless it is double-quoted.  Also,
upgrade the routine's handling of double quotes to match the backend,
in particular support doubled double quotes within quoted identifiers.
Per pgsql-interfaces discussion a couple weeks ago.