PDO: improve PDO::ATTR_STRINGIFY_FETCHES description (#4359)

Author: pereorga

reference/pdo/constants.xml

@@ -580,7 +580,9 @@ if ($db->getAttribute(PDO::ATTR_DRIVER_NAME) == 'mysql') {
    </term>
    <listitem>
     <simpara>
-     Forces all values fetched to be treated as strings.
+     Forces all fetched values (except &null;) to be treated as strings.
+     &null; values remain unchanged unless <constant>PDO::ATTR_ORACLE_NULLS</constant>
+     is set to <constant>PDO::NULL_TO_STRING</constant>.
     </simpara>
    </listitem>
   </varlistentry>

reference/pdo/pdo/setattribute.xml

@@ -136,9 +136,11 @@
      <term><constant>PDO::ATTR_STRINGIFY_FETCHES</constant></term>
      <listitem>
       <para>
-       Whether to convert numeric values to strings when fetching.
-       Takes a value of type <type>bool</type>: &true; to enable and
-       &false; to disable. <!-- By default, ??? TODO. -->
+       Controls whether fetched values (except &null;) are converted to strings.
+       Takes a value of type <type>bool</type>: &true; to enable and &false; to
+       disable (default).
+       &null; values remain unchanged unless <constant>PDO::ATTR_ORACLE_NULLS</constant>
+       is set to <constant>PDO::NULL_TO_STRING</constant>.
       </para>
      </listitem>
     </varlistentry>