Document underflow limitation in Randomizer::getFloat()

Author: lacatoire

reference/random/random/randomizer/getfloat.xml

@@ -341,6 +341,15 @@ Lat: +69.244304 Lng: -53.548951
     to obtain the desired behavioral properties.
    </para>
   </note>
+  <caution>
+   <para>
+    Underflow is intentionally left unhandled in the γ-section algorithm.
+    This may result in incorrect values being returned for intervals with
+    boundaries in the subnormal range of floating point numbers, i.e.
+    for values smaller than approximately
+    <literal>2<superscript>-1020</superscript></literal> (about <literal>8.9e-308</literal>).
+   </para>
+  </caution>
  </refsect1>
 
  <refsect1 role="seealso">