Tracing docs fix configure option and description

Fix the example trace configure option.
Update the text to say that multiple backends are allowed and what
happens when multiple backends are enabled.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 1412691161-31785-1-git-send-email-dgilbert@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

docs/tracing.txt

@@ -139,12 +139,12 @@ events are not tightly coupled to a specific trace backend, such as LTTng or
 SystemTap.  Support for trace backends can be added by extending the "tracetool"
 script.
 
-The trace backend is chosen at configure time and only one trace backend can
-be built into the binary:
+The trace backends are chosen at configure time:
 
-    ./configure --trace-backends=simple
+    ./configure --enable-trace-backends=simple
 
 For a list of supported trace backends, try ./configure --help or see below.
+If multiple backends are enabled, the trace is sent to them all.
 
 The following subsections describe the supported trace backends.