Sækja usage 7.1.5

Fixed
- (spec) A relationship property such as conflicts="--b --c" is now rejected when the spec loads (#1599, @jdx). Previously, --b --c was read as one selector that matched no flag, so --a --b and --a --c both parsed without error. Repeating the property (conflicts="--b" conflicts="--c") also loaded, but only the last value was kept. Both forms now produce an error:
conflicts takes one flag or argument per property, but "--b --c" has several; write them as a child node instead: conflicts "--a" "--b"
conflicts is given more than once; the second would replace the first, so write several as a child node instead: conflicts "--a" "--b"
This check applies to conflicts, overrides, requires, required_if, required_unless and required_unless_all on flags, and to the same properties except overrides on positional args. A value is refused only if it has more than one word and at least one starts with a dash. Values like conflicts="input file" still refer to a positional named [input file].
Upgrading: These specs were never enforcing their relationships, so they now need to be rewritten in the child-node form. That form already worked and has not changed:
flag "--a" {
conflicts "--b" "--c"
}
The flag reference (docs/spec/reference/flag.md) now covers the child-node form and these load-time errors.