8006565: java.lang.instrument specification should make it clear that -javaagent is optional

Reviewed-by: sla, dcubed, mchung

src/share/classes/java/lang/instrument/package.html

@@ -53,8 +53,10 @@ dependent.
 <h3>Command-Line Interface</h3>
 
 <P> 
-On implementations with a command-line interface, an agent is started by
-adding this option to the command-line:
+An implementation is not required to provide a way to start agents from the
+command-line interface. On implementations that do provide a way to start agents
+from the command-line interface, an agent is started by adding this option to
+the command-line:
 <blockquote>
 <code><b>-javaagent:</b></code><i>jarpath[</i><code><b>=</b></code><i>options]</i>
 </blockquote>