Add GHES version comment to feature flag

Document that GHES support will be added when the feature ships.
Keeps the feature flag clear about its current scope (fpt/ghec only).

Author: salmanmkc

data/features/vulnerability-alerts-permission.yml

@@ -1,4 +1,6 @@
 # Vulnerability alerts permission for GITHUB_TOKEN
+# GHES support will be added when the feature ships to GHES
 versions:
   fpt: '*'
   ghec: '*'
+  # ghes: '>=3.XX' # Uncomment when vulnerability-alerts permission ships to GHES