Make wording of the directory and target properties more consitent
and complementary. Specify that the value is a ";-list" with a link
to the cmake-language(7) manual section on lists.
Generator expressions are supported in many places and are a distinct
concept worthy of their own manual page. The old builtin documentation
was previously represented by preprocessor macros to generate it into
each place that supports them. Factor out the duplicate content into a
dedicated cmake-generator-expressions manual page and reference it from
each original location.