$expand: clarify that property *paths* must not appear twice

Current text on $expand in https://docs.oasis-open.org/odata/odata/v4.01/os/part2-url-conventions/odata-v4.01-os-part2-url-conventions.html#sec_SystemQueryOptionexpand says that

A property MUST NOT appear in more than one expand item.

This is misleading if e.g. a complex type is used multiple times.

The option $expand=billToAddress/country,shipToAddress/country should be valid even though the country navigation property of the Address complex type appears twice, but within different property paths.