Add documentation and regression tests concerning rounding of numerics.

Michael Paquier, reviewed by Fabien Coelho

doc/src/sgml/datatype.sgml

@@ -612,6 +612,31 @@ NUMERIC
      equivalent.  Both types are part of the <acronym>SQL</acronym>
      standard.
     </para>
+
+    <para>
+     When rounding values, the <type>numeric</type> type rounds ties away
+     from zero, while (on most machines) the <type>real</type>
+     and <type>double precision</type> types round ties to the nearest even
+     number.  For example:
+
+<programlisting>
+SELECT x,
+  round(x::numeric) AS num_round,
+  round(x::double precision) AS dbl_round
+FROM generate_series(-3.5, 3.5, 1) as x;
+  x   | num_round | dbl_round
+------+-----------+-----------
+ -3.5 |        -4 |        -4
+ -2.5 |        -3 |        -2
+ -1.5 |        -2 |        -2
+ -0.5 |        -1 |        -0
+  0.5 |         1 |         0
+  1.5 |         2 |         2
+  2.5 |         3 |         2
+  3.5 |         4 |         4
+(8 rows)
+</programlisting>
+    </para>
    </sect2>