mark chart.spec.version as specific to HelmRepository source in example

In my opinion it is not visible enough that `chart.spec.version` is only considered for `HelmRepository` sources (and silently ignored otherwise.
I'm aware that the text below the example mentions this, but I think it would be benefitial for the code example itself to make that fact clear.

Adding this comment is only a suggestion (which I'm not 100% happy with myself) -- feel free to substitute a different way of pointing out the connection between `HelmRepository` and the version spec.

content/en/docs/guides/helmreleases.md

@@ -207,7 +207,7 @@ spec:
   chart:
     spec:
       chart: <name|path>
-      version: '4.0.x'
+      version: '4.0.x'  # only when using HelmRepository
       sourceRef:
         kind: <HelmRepository|GitRepository|Bucket>
         name: podinfo