docs: update self-protection __ro_after_init status

__ro_after_init is no longer new and under development. Update the
document to reflect the correct status.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20211208164051.8783-1-skhan@linuxfoundation.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/security/self-protection.rst

@@ -81,8 +81,7 @@ of the kernel, gaining the protection of the kernel's strict memory
 permissions as described above.
 
 For variables that are initialized once at ``__init`` time, these can
-be marked with the (new and under development) ``__ro_after_init``
+be marked with the ``__ro_after_init`` attribute.
-attribute.
 
 What remains are variables that are updated rarely (e.g. GDT). These
 will need another infrastructure (similar to the temporary exceptions