rust: io: move MIN_SIZE and io_addr_assert to IoKnownSize

rust: io: move MIN_SIZE and io_addr_assert to IoKnownSize

`MIN_SIZE` and `io_addr_assert` are only ever used for IO types which
implement `IoKnownSize` and do not make sense for types that don't.

It looks like they should have been there since the beginning, so move
them while the code is still fresh.

Also update `IoKnownSize`'s documentation since it is not just a marker
trait anymore.

Fixes: 121d87b28e1d ("rust: io: separate generic I/O helpers from MMIO implementation")
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20260130-io-min-size-v1-1-65a546e3104d@nvidia.com
[ Fix typo in commit message. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>