[libc++][format] Enables feature-test macro.

The macro is only enabled when the Clang is used with
-fexperimental-library.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D130792

NOKEYCHECK=True
GitOrigin-RevId: 679169b7dd74c6f9161353f07e716ec25e80d1b8
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 7ab2d6c..9aa183a 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -234,7 +234,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_execution``                           *unimplemented*
     ------------------------------------------------- -----------------
-    ``__cpp_lib_format``                              *unimplemented*
+    ``__cpp_lib_format``                              ``202106L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_generic_unordered_lookup``            ``201811L``
     ------------------------------------------------- -----------------