Add an example of using a regex with negative lookup in --output_filter.

I promised to add this example after responding to a user question in the bazel irc.

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MOS_MIGRATED_REVID=102120516
diff --git a/site/docs/bazel-user-manual.html b/site/docs/bazel-user-manual.html
index 2ce36ea..4aa6535 100644
--- a/site/docs/bazel-user-manual.html
+++ b/site/docs/bazel-user-manual.html
@@ -1221,6 +1221,10 @@
     <td>Show the output for the specified packages.</td>
   </tr>
   <tr>
+    <td><code class='flag'>--output_filter='^//((?!(first/bad_project|second/bad_project):).)*$'</code></td>
+    <td>Don't show output for the specified packages.</td>
+  </tr>
+  <tr>
     <td><code class='flag'>--output_filter=DONT_MATCH_ANYTHING</code></td>
     <td>Don't show output.</td>
   </tr>