Help: Re-organize and clarify add_library docs
Re-order the add_library
documentation sections to put in-project target types first and imported/alias target types last.
Move IMPORTED INTERFACE library documentation into the main Imported Libraries section. Revise the latter section to more clearly document the properties needed to describe each type of imported library.
While at it, tweak target_sources
documentation to clarify the PUBLIC/PRIVATE/INTERFACE scopes.