mention epicsEnvUnset in RELEASE_NOTES.html

documentation/RELEASE_NOTES.html

@@ -16,6 +16,10 @@
 
 <!-- Insert new items immediately below here ... -->
 
+<h3>Unsetting environment variables</h3>
+The new command <code>epicsEnvUnset <i>varname</i></code> can be used to
+unset an environment variable.
+
 <h3>Fix problem with numeric soft events</h3>
 <p>Changing from numeric to named soft events introduced an incompatibility
 when a numeric event 1-255 is converted from a DOUBLE, e.g. from a calc record.