Update GCE discovery documentation
Update GCE discovery documentation

(cherry picked from commit d4cb81c)
(cherry picked from commit 9af04eb)

strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test