Add reciprocal links section to data-not-migrated.md (#61835)

Author: jfine

data/reusables/enterprise-migration-tool/data-not-migrated.md

@@ -14,6 +14,7 @@
 * Mentions of users, teams, and organizations in pull request, issue, release, and comment bodies (the username originally mentioned is retained)
 * Packages in {% data variables.product.prodname_registry %}
 * {% data variables.product.prodname_projects_v2 %} (the new projects experience)
+* Reciprocal links from mentions of issues, pull requests, discussions, teams, or milestones
 * References between pull requests and issues in different repositories (see [AUTOTITLE](/get-started/writing-on-github/working-with-advanced-formatting/autolinked-references-and-urls))
 * Remediation states of {% data variables.product.prodname_secret_scanning %} results
 * Repositories owned by user accounts