Mention extra_float_digits in floating point docs

Make it easier for readers of the FP docs to find out about possibly
truncated values.

Per complaint from Tom Duffey in message
F0E0F874-C86F-48D1-AA2A-0C5365BF5118@trillitech.com

Author: Albe Laurenz
Reviewed by: Abhijit Menon-Sen

2 files changed, 12 insertions, 0 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index ab40ea76122..088473a8286 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -4739,6 +4739,7 @@ SET XML OPTION { DOCUMENT | CONTENT };
         partially-significant digits; this is especially useful for dumping
         float data that needs to be restored exactly.  Or it can be set
         negative to suppress unwanted digits.
+        See also <xref linkend="datatype-float">.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index 0e07cc35eac..f15a47a2f4a 100644
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -663,6 +663,17 @@ NUMERIC
      from zero will cause an underflow error.
     </para>
 
+    <note>
+     <para>
+      The <xref linkend="guc-extra-float-digits"> setting controls the
+      number of extra significant digits included when a floating point
+      value is converted to text for output.  With the default value of
+      <literal>0</literal>, the output is the same on every platform
+      supported by PostgreSQL.  Increasing it will produce output that
+      more accurately represents the stored value, but may be unportable.
+     </para>
+    </note>
+
     <indexterm>
      <primary>not a number</primary>
      <secondary>double precision</secondary>