Documentation/x86: Update documentation for SVA (Shared Virtual Addressing)

Adjust the documentation to the new way how a PASID is being allocated,
freed and fixed up.

Based on a patch by Ashok Raj <ashok.raj@intel.com>

  [ bp: Massage commit message, fix htmldocs build warning ]

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20220207230254.3342514-12-fenghua.yu@intel.com