Apply suggestions from code review for GTEST_SKIP documentation

Extended example and some rewording by @ericschmidtatwork, thank you.

Co-authored-by: Eric Schmidt <shibumi@google.com>

docs/advanced.md

@@ -551,7 +551,7 @@ TEST_F(Fixture, SkipsOneTest) {
 }
 ```
 
-The informational text is optional.
+As with assertion macros, you can stream a custom message into `GTEST_SKIP()`.
 
 ## Teaching googletest How to Print Your Values