Add core badges to admin_area docs

Some admin_area docs were missing the Core Only badge

doc/user/admin_area/abuse_reports.md

@@ -2,7 +2,7 @@
 type: reference, howto
 ---
-# Abuse reports
+# Abuse reports **(CORE ONLY)**
 View and resolve abuse reports from GitLab users.

doc/user/admin_area/broadcast_messages.md

@@ -2,7 +2,7 @@
 type: reference, howto
 ---
-# Broadcast Messages
+# Broadcast Messages **(CORE ONLY)**
 GitLab can display messages to all users of a GitLab instance in a banner that appears in the UI.

doc/user/admin_area/diff_limits.md

@@ -2,7 +2,7 @@
 type: reference
 ---
-# Diff limits administration
+# Diff limits administration **(CORE ONLY)**
 You can set a maximum size for display of diff files (patches).

doc/user/admin_area/monitoring/health_check.md

@@ -2,7 +2,7 @@
 type: concepts, howto
 ---
-# Health Check
+# Health Check **(CORE ONLY)**
 > - Liveness and readiness probes were [introduced][ce-10416] in GitLab 9.1.
 > - The `health_check` endpoint was [introduced][ce-3888] in GitLab 8.8 and was

doc/user/admin_area/settings/account_and_limit_settings.md

@@ -2,7 +2,7 @@
 type: reference
 ---
-# Account and limit settings
+# Account and limit settings **(CORE ONLY)**
 ## Max attachment size

doc/user/admin_area/settings/email.md

@@ -2,7 +2,7 @@
 type: reference
 ---
-# Email
+# Email **(CORE ONLY)**
 You can customize some of the content in emails sent from your GitLab instance.

doc/user/admin_area/settings/sign_up_restrictions.md

@@ -2,7 +2,7 @@
 type: reference
 ---
-# Sign-up restrictions
+# Sign-up restrictions **(CORE ONLY)**
 You can use sign-up restrictions to require user email confirmation, as well as
 to blacklist or whitelist email addresses belonging to specific domains.

doc/user/admin_area/settings/terms.md

@@ -2,7 +2,7 @@
 type: reference
 ---
-# Enforce accepting Terms of Service
+# Enforce accepting Terms of Service **(CORE ONLY)**
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18570)
 > in [GitLab Core](https://about.gitlab.com/pricing/) 10.8

doc/user/admin_area/settings/third_party_offers.md

@@ -2,7 +2,7 @@
 type: reference
 ---
-# Third party offers
+# Third party offers **(CORE ONLY)**
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20379)
 > in [GitLab Core](https://about.gitlab.com/pricing/) 11.1

doc/user/admin_area/settings/usage_statistics.md

@@ -2,7 +2,7 @@
 type: reference
 ---
-# Usage statistics
+# Usage statistics **(CORE ONLY)**
 GitLab Inc. will periodically collect information about your instance in order
 to perform various actions.

doc/user/admin_area/settings/user_and_ip_rate_limits.md

@@ -2,7 +2,7 @@
 type: reference
 ---
-# User and IP rate limits
+# User and IP rate limits **(CORE ONLY)**
 Rate limiting is a common technique used to improve the security and durability
 of a web application. For more details, see

doc/user/admin_area/settings/visibility_and_access_controls.md

@@ -2,7 +2,7 @@
 type: reference
 ---
-# Visibility and access controls
+# Visibility and access controls **(CORE ONLY)**
 GitLab allows administrators to: