Fix documentation for libpq's PQfn().

The SGML docs claimed that 1-byte integers could be sent or received with
the "isint" options, but no such behavior has ever been implemented in
pqGetInt() or pqPutInt().  The in-code documentation header for PQfn() was
even less in tune with reality, and the code itself used parameter names
matching neither the SGML docs nor its libpq-fe.h declaration.  Do a bit
of additional wordsmithing on the SGML docs while at it.

Since the business about 1-byte integers is a clear documentation bug,
back-patch to all supported branches.