Update default entities admin UI (FusionAuth#2921)

* Show that AS validates credentials

* change wording to be more clear

astro/src/content/docs/_shared/_default-entities.mdx

@@ -1,5 +1,5 @@
 * The Default tenant, which will always exist.
-* The FusionAuth application, which resides in the Default tenant. This will always have the Id `3c219e58-ed0e-4b18-ad48-f4f92793ae32`.
+* The FusionAuth application, which resides in the Default tenant. This will always have the Id `3c219e58-ed0e-4b18-ad48-f4f92793ae32`. This is also known as the administrative user interface or admin UI.
 * The Default theme, which is always read only. This will always have the Id `75a068fd-e94b-451a-9aeb-3ddb9a3b5987`.
 * The FusionAuth application roles, which may be assigned to users, but are read only. View the [roles documentation](/docs/get-started/core-concepts/roles) for the immutable Ids of each role.
 * Three OpenID Connect shadow keys. These may not be modified or removed. When any of these keys are selected as the signing key for the Id Token, the Client Secret defined by the FusionAuth application will be used to sign the token. This is per [section 10.1 of the OIDC specification](https://openid.net/specs/openid-connect-core-1_0.html#rfc.section.10.1).