docs: add a note that attr 'managed' is only used by PCI devices

docs: add a note that attr 'managed' is only used by PCI devices

Our documentation isn't 100% clear about hostdev 'managed' attribute usage,
because it only makes sense to use it with PCI devices, yet we format
this attribute to all hostdev devices. By adding a note into the docs,
we can possibly avoid confusion from customer's side and also avoid a solution
using ternary logic.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1155887