Clarify GHAS billing migration benefit requires GitHub Enterprise Importer (#58853)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: felickz <1760475+felickz@users.noreply.github.com>
Co-authored-by: heiskr <1221423+heiskr@users.noreply.github.com>
Co-authored-by: mc <42146119+mchammer01@users.noreply.github.com>

Author: 4 people

content/billing/concepts/product-billing/github-advanced-security.md

@@ -53,7 +53,7 @@ Each **active committer** to at least one repository with an {% data variables.p
 * **Active committers** are committers who contributed to at least one repository  and have a {% data variables.product.prodname_team %} or {% data variables.product.prodname_enterprise %} license with your organization or enterprise. That is, they are also a member, an enterprise-managed user, an external collaborator, or have a pending invitation to join your organization or enterprise.
 * **Unique committers** is the number of active committers who contributed only to one repository, or only to repositories in one organization. You can free up this number of licenses by disabling {% data variables.product.prodname_GH_cs_or_sp %} for that repository or organization.
 
-> [!NOTE] When a repository is migrated to GitHub, GitHub Advanced Security only consumes licenses for commits and pushes made after migration, rather than considering all historic contributions from before the migration.
+> [!NOTE] When a repository is migrated to {% data variables.product.github %} using {% data variables.product.prodname_importer_proper_name %}, {% data variables.product.prodname_GHAS %} only consumes licenses for commits and pushes made _after_ migration. Historic contributions from _before_ the migration are not considered. For more information, see [AUTOTITLE](/migrations/using-github-enterprise-importer/understanding-github-enterprise-importer/about-github-enterprise-importer).
 
 {% ifversion security-configurations %}You can see the active and unique committers to an organization on the Global settings page for {% data variables.product.UI_advanced_security %}. Under "{% data variables.product.prodname_secret_protection %} repositories" and "{% data variables.product.prodname_code_security %} repositories", summaries and repository-level details are reported. See [AUTOTITLE](/code-security/securing-your-organization/enabling-security-features-in-your-organization/configuring-global-security-settings-for-your-organization).{% endif %}