Add documentation note that getAccessToken() returns cached value (#162)

Author: chingor13

google-auth-library-java/oauth2_http/java/com/google/auth/oauth2/OAuth2Credentials.java

@@ -37,7 +37,6 @@
 import com.google.auth.http.AuthHttpConstants;
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.MoreObjects;
-import com.google.common.base.MoreObjects.ToStringHelper;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.Iterables;
 
@@ -131,6 +130,11 @@ public boolean hasRequestMetadataOnly() {
     return true;
   }
 
+  /**
+   * Returns the cached access token.
+   *
+   * <p>If not set, you should call {@link #refresh()} to fetch and cache an access token.</p>
+   */
   public final AccessToken getAccessToken() {
     return temporaryAccess;
   }