[SYCL][NFC] Move root group extension to experimental (#12819)

Signed-off-by: Michael Aziz <[email protected]>

Author: 0x12CC

sycl/doc/extensions/proposed/sycl_ext_oneapi_root_group.asciidoc renamed to sycl/doc/extensions/experimental/sycl_ext_oneapi_root_group.asciidoc

@@ -52,11 +52,12 @@ This extension also depends on the following other SYCL extensions:
 
 == Status
 
-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.*
+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.
+*Shipping software products should not rely on APIs defined in this
+specification.*
 
 
 == Overview