Apply the changes from commit 603fe925 (Fix summary documentation of
INTERFACE_LINK_LIBRARIES, 2013-11-04) and commit 0aa97b6b (Fix spelling
in INTERFACE_LINK_LIBRARIES documentation, 2013-11-04) to the new
location of 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.