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.

doc/src/sgml/libpq.sgml

@@ -4810,22 +4810,29 @@ typedef struct
    parameters to be passed to the function; they must match the declared
    function argument list.  When the <parameter>isint</> field of a
    parameter structure is true, the <parameter>u.integer</> value is sent
-   to the server as an integer of the indicated length (this must be 1,
-   2, or 4 bytes); proper byte-swapping occurs.  When <parameter>isint</>
+   to the server as an integer of the indicated length (this must be
+   2 or 4 bytes); proper byte-swapping occurs.  When <parameter>isint</>
    is false, the indicated number of bytes at <parameter>*u.ptr</> are
    sent with no processing; the data must be in the format expected by
    the server for binary transmission of the function's argument data
-   type.  <parameter>result_buf</parameter> is the buffer in which to
-   place the return value.  The caller must  have  allocated sufficient
+   type.  (The declaration of <parameter>u.ptr</> as being of
+   type <type>int *</> is historical; it would be better to consider
+   it <type>void *</>.)
+   <parameter>result_buf</parameter> points to the buffer in which to place
+   the function's return value.  The caller must have allocated sufficient
    space to store the return value.  (There is no check!) The actual result
-   length will be returned in the integer pointed to  by
-   <parameter>result_len</parameter>.  If a 1, 2, or 4-byte integer result
+   length in bytes will be returned in the integer pointed to by
+   <parameter>result_len</parameter>.  If a 2- or 4-byte integer result
    is expected, set <parameter>result_is_int</parameter> to 1, otherwise
    set it to 0.  Setting <parameter>result_is_int</parameter> to 1 causes
    <application>libpq</> to byte-swap the value if necessary, so that it
-   is delivered as a proper <type>int</type> value for the client machine.
+   is delivered as a proper <type>int</type> value for the client machine;
+   note that a 4-byte integer is delivered into <parameter>*result_buf</>
+   for either allowed result size.
    When <parameter>result_is_int</> is 0, the binary-format byte string
-   sent by the server is returned unmodified.
+   sent by the server is returned unmodified. (In this case it's better
+   to consider <parameter>result_buf</parameter> as being of
+   type <type>void *</>.)
   </para>
 
   <para>

src/interfaces/libpq/fe-exec.c

@@ -2514,20 +2514,18 @@ PQendcopy(PGconn *conn)
  *		PQfn -	Send a function call to the POSTGRES backend.
  *
  *		conn			: backend connection
- *		fnid			: function id
- *		result_buf		: pointer to result buffer (&int if integer)
- *		result_len		: length of return value.
- *		actual_result_len: actual length returned. (differs from result_len
- *						  for varlena structures.)
- *		result_type		: If the result is an integer, this must be 1,
+ *		fnid			: OID of function to be called
+ *		result_buf		: pointer to result buffer
+ *		result_len		: actual length of result is returned here
+ *		result_is_int	: If the result is an integer, this must be 1,
  *						  otherwise this should be 0
- *		args			: pointer to an array of function arguments.
+ *		args			: pointer to an array of function arguments
  *						  (each has length, if integer, and value/pointer)
  *		nargs			: # of arguments in args array.
  *
  * RETURNS
  *		PGresult with status = PGRES_COMMAND_OK if successful.
- *			*actual_result_len is > 0 if there is a return value, 0 if not.
+ *			*result_len is > 0 if there is a return value, 0 if not.
  *		PGresult with status = PGRES_FATAL_ERROR if backend returns an error.
  *		NULL on communications failure.  conn->errorMessage will be set.
  * ----------------
@@ -2537,12 +2535,12 @@ PGresult *
 PQfn(PGconn *conn,
 	 int fnid,
 	 int *result_buf,
-	 int *actual_result_len,
+	 int *result_len,
 	 int result_is_int,
 	 const PQArgBlock *args,
 	 int nargs)
 {
-	*actual_result_len = 0;
+	*result_len = 0;
 
 	if (!conn)
 		return NULL;
@@ -2560,12 +2558,12 @@ PQfn(PGconn *conn,
 
 	if (PG_PROTOCOL_MAJOR(conn->pversion) >= 3)
 		return pqFunctionCall3(conn, fnid,
-							   result_buf, actual_result_len,
+							   result_buf, result_len,
 							   result_is_int,
 							   args, nargs);
 	else
 		return pqFunctionCall2(conn, fnid,
-							   result_buf, actual_result_len,
+							   result_buf, result_len,
 							   result_is_int,
 							   args, nargs);
 }