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.

Note that this documents the desired requirements. devctl_safe_quote()
doesn't yet escape backslash characters, and devd doesn't handle escaped
characters in quoted values.

Differential Revision: https://reviews.freebsd.org/D6252