Improve description for filter_files

Co-authored-by: Michał Szczepanik <[email protected]>

Author: yarikoptic

docs/heuristics.rst

@@ -85,6 +85,8 @@ A utility function used to filter any input files.
 If this function is included, every file found will go through this filter. Any
 files where this function returns ``False`` will be filtered out (files where
 this function returns ``True`` will be kept).
+Note: this function's logic is opposite to ``filter_dicom`` (see
+below). Think of it as an input for Python's built-in ``filter()``.
 
 --------------------------
 ``filter_dicom(dcm_data)``