update advice for --prune flag (sourcegraph#783)

* update advice for --prune flag

Co-authored-by: uwedeportivo <[email protected]>

Author: davejrt

kubectl-apply-all.sh

@@ -5,6 +5,10 @@
 # This file should be run:
 #   * When the cluster is first created
 #   * Whenever the configuration for any resource has been updated
-
+#
+# The --prune flag is destructive and should always be used 
+# in conjunction with -f base and -l deploy=sourcegraph. Otherwise, it will delete all resources 
+# previously created by create or apply that are not specified in the command.
+#
 # Apply the base Soucegraph deployment
 kubectl apply --prune -l deploy=sourcegraph -f base --recursive $@