[2252] update descriptions

consider an exceptional case that transferring is done in less than a
microsecond

src/bin/xfrin/b10-xfrin.xml

@@ -297,14 +297,16 @@ operation
                   <varlistentry>
                     <term>last_ixfr_duration</term>
                     <listitem><simpara>
-                      Duration of the last IXFR. 0.0 means no successful IXFR done.
+                      Duration of the last IXFR. 0.0 means no successful IXFR done
+                      in greater than or equal to a microsecond.
                     </simpara></listitem>
                   </varlistentry>
 
                   <varlistentry>
                     <term>last_axfr_duration</term>
                     <listitem><simpara>
-                      Duration of the last AXFR. 0.0 means no successful AXFR done.
+                      Duration of the last AXFR. 0.0 means no successful AXFR done
+                      in greater than or equal to a microsecond.
                     </simpara></listitem>
                   </varlistentry>
 

src/bin/xfrin/xfrin.spec

@@ -194,7 +194,7 @@
               "item_optional": false,
               "item_default": 0.0,
               "item_title": "Last IXFR duration",
-              "item_description": "Duration of the last IXFR. 0.0 means no successful IXFR done."
+              "item_description": "Duration of the last IXFR. 0.0 means no successful IXFR done in greater than or equal to a microsecond."
             },
             {
               "item_name": "last_axfr_duration",
@@ -202,7 +202,7 @@
               "item_optional": false,
               "item_default": 0.0,
               "item_title": "Last AXFR duration",
-              "item_description": "Duration of the last AXFR. 0.0 means no successful AXFR done."
+              "item_description": "Duration of the last AXFR. 0.0 means no successful AXFR done in greater than or equal to a microsecond."
             }
           ]
         }