Merge topic 'source_file_both_props' into release-3.18

961ee62faa Help: Update get_property and get_source_file_property docs
1235f2d747 set_property: Allow both DIRECTORY and TARGET_DIRECTORY together
177052d6b8 set_property: Fix name of TARGET_DIRECTORY option in error messages

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !4994