docs: move -xn flag beside --nonmoving-gc

It makes sense to have these beside each other as they are aliases.

(cherry picked from commit 98166389)

docs/users_guide/runtime_control.rst

@@ -411,6 +411,13 @@ performance.
     Note that :rts-flag:`--nonmoving-gc` cannot be used with ``-G1``,
     :rts-flag:`profiling <-hc>` nor :rts-flag:`-c`.
 
+.. rts-flag:: -xn
+
+    :default: off
+    :since: 8.10.1
+
+    An alias for :rts-flag:`--nonmoving-gc`
+
 .. rts-flag:: -w
 
     :default: off
@@ -422,13 +429,6 @@ performance.
     (:rts-flag:`-hT`) unless linked against the profiling runtime system with
     :ghc-flag:`-prof`.
 
-.. rts-flag:: -xn
-
-    :default: off
-    :since: 8.10.1
-
-    An alias for :rts-flag:`--nonmoving-gc`
-
 .. rts-flag:: -A ⟨size⟩
 
     :default: 4MB