x86/mm/doc: Add missing details in virtual memory layout

Improve memory layout documentation:

 - Document 4kB guard hole at the end of userspace.
   See TASK_SIZE_MAX definition.

 - Divide the description of the non-canonical hole into two parts:
   userspace and kernel sides.

 - Mention the effect of LAM on the non-canonical range.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20241031084946.2243440-1-kirill.shutemov%40linux.intel.com