doc: update NUMERIC to state that only some ops are exact

Report by Tom Lane

diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index edf636b441758904b9d013023a1a066b1eedfc38..da1f25fe285e731d73793043f72eb8ec45ae0bcc 100644 (file)
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -498,11 +498,13 @@
 
     
      The type numeric can store numbers with a
-     very large number of digits and perform calculations exactly. It is
-     especially recommended for storing monetary amounts and other
-     quantities where exactness is required. However, arithmetic on
-     numeric values is very slow compared to the integer
-     types, or to the floating-point types described in the next section.
+     very large number of digits. It is especially recommended for
+     storing monetary amounts and other quantities where exactness is
+     required.  Calculations with numeric values yield exact
+     results where possible, e.g.  addition, subtraction, multiplication.
+     However, calculations on numeric values are very slow
+     compared to the integer types, or to the floating-point types
+     described in the next section.