Features: Add missing variable reference in docs.

Help/prop_tgt/CXX_EXTENSIONS.rst

@@ -6,3 +6,7 @@ Boolean specifying whether compiler specific extensions are requested.
 This property specifies whether compiler specific extensions should be
 used.  For some compilers, this results in adding a flag such
 as ``-std=gnu++11`` instead of ``-std=c++11`` to the compile line.
+
+This property is initialized by the value of
+the :variable:`CMAKE_CXX_EXTENSIONS` variable if it is set when a target
+is created.

Help/prop_tgt/C_EXTENSIONS.rst

@@ -6,3 +6,7 @@ Boolean specifying whether compiler specific extensions are requested.
 This property specifies whether compiler specific extensions should be
 used.  For some compilers, this results in adding a flag such
 as ``-std=gnu11`` instead of ``-std=c11`` to the compile line.
+
+This property is initialized by the value of
+the :variable:`CMAKE_C_EXTENSIONS` variable if it is set when a target
+is created.