document what the 'Age' value in the AltosUI display means

diff --git a/doc/altusmetrum.xsl b/doc/altusmetrum.xsl
index e7ab353bbd2ef171107f6f937048cc476845a657..0fd19e60639eb40bd98c3c5a0c108243d147e422 100644 (file)
--- a/doc/altusmetrum.xsl
+++ b/doc/altusmetrum.xsl
@@ -769,6 +769,15 @@ NAR #88757, TRA #12200
             incorrect data from being reported.
           </para>
         </listitem>
+        <listitem>
+          <para>
+            The age of the displayed data, in seconds since the last 
+           successfully received telemetry packet.  In normal operation
+           this will stay in the low single digits.  If the number starts
+           counting up, then you are no longer receiving data over the radio
+           link from the flight computer.
+          </para>
+        </listitem>
       </itemizedlist>
       <para>
         Finally, the largest portion of the window contains a set of