Document the && syntax for property ordering.

[jurko.gospodnetic@…]

Document the && syntax for specifying the order in which multiple property values should be applied.

For example: <include>bbb&&aaa adds the bbb folder to the include path first and adds the aaa folder afterwards while <include>bbb <include>aaa does not make this guarantee (and effectively does not do this in the current implementation due to all properties getting sorted alphabetically internally.