DOC: fix the description of filter_files

The filter_files is used in the reproin heuristic (where it always
returns True), with this docstring: "Return True if a file should be
kept, else False". This commit adjusts the docs to effectively state
the same (it used to say the opposite).

The docstring in the heudiconv heuristic was right: filter_files is
fed to `dicoms.py::group_dicoms_into_seqinfos()` as `file_filter`,
where it is applied with built-in `filter()` to the to-be-processed
file list; `filter()` yields items for which `function(item) == True`.

N.B. This change makes it clear that `filter_files` and `filter_dicom`
have opposite logic. The latter is fed to
`dicoms.py::group_dicoms_into_seqinfos()` as `dcmfilter`; `dcmfilter`
is passed into `dicoms.py::validate_dicom()`; validate_dicom returns
`None` (meaning: *not valid*) if `dcmfilter(mw.dcm_data)` is `True`.

So for a file to be kept, `filter_files` must return `True`, and
`filter_dicom` must return `False`. This commit changes the
documentation rather than the logic, assuming that the logic has been
present long enough to warrant preservation (it is also easier to only
change the docs).

Author: mslw

docs/heuristics.rst

@@ -83,7 +83,8 @@ But it is not used directly by HeuDiConv.
 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 ``True`` will be filtered out.
+files where this function returns ``False`` will be filtered out (files where
+this function returns ``True`` will be kept).
 
 --------------------------
 ``filter_dicom(dcm_data)``