Make the documentation change made by commit ef10b87c (CMP0022: Plain
target_link_libraries must populate link interface, 2013-11-02) in the
new location for the same documentation.
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.
