Documentation/x86: Add documentation for using dynamic XSTATE features

Documentation/x86: Add documentation for using dynamic XSTATE features

BugLink: https://bugs.launchpad.net/bugs/1967750
Explain how dynamic XSTATE features can be enabled via the
architecture-specific prctl() along with dynamic sigframe size and
first use trap handling.

Fix:

Documentation/x86/xstate.rst:15: WARNING: Title underline too short.

as reported by Stephen Rothwell <sfr@canb.auug.org.au>

Originally-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211026091157.16711-1-chang.seok.bae@intel.com
(cherry picked from commit d7a9590f608dbedd917eb0857a074accdf0d3919)
Acked-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>