Document the formatting requirements of location and pnpinfo strings.
devd requires location and pnpinfo strings generated by bus drivers
to be formatted as a list of name=value keypairs. Non-conforming
bus drivers cause devd to mis-parse device events for these buses.
For example, PCI bus events have been broken since the location string
was changed to use a bare 'pciD.B.S.F' as the location string.