docs/devel: Mention post_load hook restrictions where we document the hook

docs/devel: Mention post_load hook restrictions where we document the hook

Accessing another device in a post_load hook is a bad idea, because
the order of device save/restore is not fixed, and so this
cross-device access makes the save/restore non-deterministic.

We previously only flagged up this requirement in the
record-and-replay developer docs; repeat it in the main migration
documentation, where a developer trying to implement a post_load hook
is more likely to see it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>