Clarifies how the test_filter flag filters.
@@ -1229,7 +1229,8 @@ memory-saving mode</a>.
<h4 id="flag--test_filter"><code class='flag'>--test_filter=<var>filter-expression</var></code></h4>
<p>
Specifies a filter that the test runner may use to pick a subset of tests for
- running. All targets specified in the invocation are built, but depending on
+ running. See <a href="http://go/testfilter">go/testfilter</a> for how the filter does matching for your language.
+ All targets specified in the invocation are built, but depending on
the expression only some of them may be executed; in some cases, only certain
test methods are run.
</p>
