g.version manual: explain "revision=exported"

general/g.version/g.version.html

@@ -69,6 +69,9 @@ geos=3.7.1
 sqlite=3.26.0
 </pre></div>
 
+Note: if <tt>revision=exported</tt> is reported instead of the git hash then the
+<tt>git</tt> program was not available during compilation of GRASS GIS.
+
 <h2>Citing GRASS GIS</h2>
 
 The GRASS Development Team has invested significant time and effort