Docs: Document LDAP config reload in admin http api (#18739)

Fixes #17221

docs/sources/http_api/admin.md

@@ -488,3 +488,30 @@ Content-Type: application/json
   "message": "Dashboards config reloaded"
 }
 ```
+
+## Reload LDAP configuration
+
+`POST /api/admin/ldap/reload`
+
+Reloads the LDAP configuration.
+
+Only works with Basic Authentication (username and password). See [introduction](http://docs.grafana.org/http_api/admin/#admin-api) for an explanation.
+
+**Example Request**:
+
+```http
+POST /api/admin/ldap/reload HTTP/1.1
+Accept: application/json
+Content-Type: application/json
+```
+
+**Example Response**:
+
+```http
+HTTP/1.1 200
+Content-Type: application/json
+
+{
+  "message": "LDAP config reloaded"
+}
+```