Add description of placeholders which could be used in the produced t…

…emplates

docs/heuristics.rst

@@ -28,10 +28,24 @@ Conversion outputs are defined as keys, a `tuple` consisting of three elements:
 - `None` - a historical artifact (corresponds to some notion of
   ``annotation_class`` no living human is aware about)
 
+The following details of the sequences could also be used as a ``{detail}`` in the conversion keys:
+
+- ``item``: an index of seqinfo (e.g., ``1``),
+- ``subject``: a subject label (e.g., ``qa``)
+- ``seqitem``: sequence item, index with a sequence/protocol name (e.g., ``3-anat-scout_ses-{date}``)
+- ``subindex``: an index within the ``seqinfo`` (e.g., ``1``),
+- ``session``: empty (no session) or a session entity (along with ``ses-``, e.g., ``ses-20191216``),
+- ``bids_subject_session_prefix``: shortcut for BIDS file name prefix combining subject and optional session (e.g., ``sub-qa_ses-20191216``),
+- ``bids_subject_session_dir``: shortcut for BIDS file path combining subject and optional session (e.g., ``sub-qa/ses-20191216``).
+
 .. note:: An example conversion key
 
     ``('sub-{subject}/func/sub-{subject}_task-test_run-{item}_bold', ('nii.gz', 'dicom'), None)``
 
+	or equivalent in `--bids` mode which would work also if there is a specified session
+
+    ``('{bids_subject_session_dir}/func/{bids_subject_session_prefix}_task-test_run-{item}_bold', ('nii.gz', 'dicom'), None)``
+
 The ``seqinfos`` parameter is a list of namedtuples which serves as a grouped and
 stacked record of the DICOMs passed in. Each item in `seqinfo` contains DICOM
 metadata that can be used to isolate the series, and assign it to a conversion
@@ -49,6 +63,7 @@ A dictionary of {``conversion key``: ``series_id``} is returned, where
 ---------------------------------
 
 A common helper function used to create the conversion key in ``infotodict``.
+But it is not used directly by HeuDiConv.
 
 --------------------
 ``filter_files(fl)``