docs(link): 写明内部面明文 HTTP 的单节点前提与放行范围，修正配置模板里的内部面地址

plugins/shared/src/main/java/best/lolicon/felis/link/LinkConfigLoader.java

@@ -65,8 +65,10 @@ public final class LinkConfigLoader {
                 + "# Both values are normally injected via environment variables\n"
                 + "# (" + ENV_URL + ", " + ENV_TOKEN + "); this file is the fallback.\n"
                 + "#\n"
-                + "# " + KEY_URL + ": base URL of the felis-api internal face, e.g.\n"
-                + "#   http://felis-api.felis.svc.cluster.local:8080\n"
+                + "# " + KEY_URL + ": base URL of the felis-api internal face: the ClusterIP\n"
+                + "# of Service felis-api-internal, port 8081, e.g. http://10.43.0.10:8081\n"
+                + "# (kubectl -n felis get svc felis-api-internal). The installer writes it for\n"
+                + "# the proxy; it is reachable from the k3s node and permitted pods only.\n"
                 + KEY_URL + "=\n"
                 + "#\n"
                 + "# " + KEY_TOKEN + ": the internal service token (keep this secret).\n"