feat(STONEINTG-892): Override source explanation

Revise the explanation for the 'source' key to hopefully avoid some
people having unexpected problems getting their EC check to pass
after creating an override snapshot.

(Could be squashed with last commit if preferred.)

docs/modules/ROOT/pages/how-to-guides/testing_applications/proc_creating_override_snapshot.adoc

@@ -40,7 +40,7 @@ spec:
 <1> The label `test.appstudio.openshift.io/type: override` that indicates this is an `override` snapshot.
 <2> The component name you will reset its Global Candidate List.
 <3> The container image with valid digest you will reset the component <2> to.
-<4> Optional. The component source containing the git revision.
+<4> Optional, but may cause an Enterprise Contract check to fail if it's incorrect or missing. The component source containing the git url and revision that the container image was built from.
 
 
 . Save the .yaml file and add the snapshot.yaml by running the following command: