[libc++] Implement missing feature-test macro __cpp_lib_shared_ptr_arrays

This was forgotten when we implemented support for arrays in std::shared_ptr
in https://reviews.llvm.org/D62259.

GitOrigin-RevId: a65dc08d105e147c98ef17a47b9504210f6058c5
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index aa60a03..091d4b7 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -146,7 +146,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_shared_mutex``                        ``201505L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_shared_ptr_arrays``                   *unimplemented*
+    ``__cpp_lib_shared_ptr_arrays``                   ``201611L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_shared_ptr_weak_type``                ``201606L``
     ------------------------------------------------- -----------------