Clarify secret name casing behavior in GitHub Actions documentation (#56014)

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: am-stead <[email protected]>
Co-authored-by: elbrenn <[email protected]>

Author: 3 people

data/reusables/actions/actions-secrets-and-variables-naming.md

@@ -1,5 +1,5 @@
 * Can only contain alphanumeric characters (`[a-z]`, `[A-Z]`, `[0-9]`) or underscores (`_`). Spaces are not allowed.
 * Must not start with the `GITHUB_` prefix.
 * Must not start with a number.
-* Are case insensitive.
+* Are case insensitive when referenced. {% data variables.product.github %} stores secret names as uppercase regardless of how they are entered.
 * Must be unique to the repository, organization, or enterprise where they are created.