pgsql: Fix documentation for libpq's PQfn().

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.

Branch
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REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/a07bd32f2b130a67bf285f8ef5f1fe4ebcc3a5b8

Modified Files
--------------
doc/src/sgml/libpq.sgml | 23 +++++++++++++++--------
src/interfaces/libpq/fe-exec.c | 22 ++++++++++------------
2 files changed, 25 insertions(+), 20 deletions(-)