Restore proposed status of work group memory doc

Author: lbushi25

sycl/doc/extensions/proposed/sycl_ext_oneapi_work_group_memory.asciidoc

@@ -49,10 +49,12 @@ This extension also depends on the following other SYCL extensions:
 
 == Status
 
-This is an experimental extension specification, intended to provide early
-access to features and gather community feedback.  Interfaces defined in this
-specification are implemented in {dpcpp}, but they are not finalized and may
-change incompatibly in future versions of {dpcpp} without prior notice.
+This is a proposed extension specification, intended to gather community
+feedback.
+Interfaces defined in this specification may not be implemented yet or may be
+in a preliminary state.
+The specification itself may also change in incompatible ways before it is
+finalized.
 *Shipping software products should not rely on APIs defined in this
 specification.*