Link from Expect to ExpectFailure.

We already have a link in the ExpectationGatherer deprecation text, but the link will go away when the class is made public. And it's clear from the old ExpectationGatherer user and from CL 167017292 that people are finding Expect when they want ExpectFailure.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=167026073
diff --git a/core/src/main/java/com/google/common/truth/Expect.java b/core/src/main/java/com/google/common/truth/Expect.java
index 4c21ac8..f45ecfc 100644
--- a/core/src/main/java/com/google/common/truth/Expect.java
+++ b/core/src/main/java/com/google/common/truth/Expect.java
@@ -47,6 +47,9 @@
  *
  * If both of the assertions above fail, the test will fail with an exception that contains
  * information about both.
+ *
+ * <p>To record failures for the purpose of testing that an assertion fails when it should, see
+ * {@link ExpectFailure}.
  */
 @GwtIncompatible("JUnit4")
 public final class Expect extends StandardSubjectBuilder implements TestRule {