Documentation: ublk: address review comments for SHMEM_ZC docs

Documentation: ublk: address review comments for SHMEM_ZC docs

- Use "physical pages" instead of "page frame numbers (PFNs)" for
  clarity
- Remove "without any per-I/O overhead" claim from zero-copy
  description
- Add scatter/gather limitation: each I/O's data must be contiguous
  within a single registered buffer

Suggested-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260409133020.3780098-7-tom.leiming@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>