ioctl_userfaultfd.2, userfaultfd.2: add minor fault mode

ioctl_userfaultfd.2, userfaultfd.2: add minor fault mode

Userfaultfd minor fault mode is supported starting from Linux 5.13.

This commit adds a description of the new mode, as well as the new ioctl
used to resolve such faults. The two go hand-in-hand: one can't resolve
a minor fault without continue, and continue can't be used to resolve
any other kind of fault.

v1->v2:
- Some spelling / phrasing improvements
v2->v3:
- Improved line wrapping in man2/ioctl_userfaultfd.2
v3->v4:
- Rebased onto http://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/ main
- Mentioned minor fault shmem support
- Improved line wrapping in man2/userfaultfd.2

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>