doc: clarify bids split naming (#12948)

mne/utils/docs.py

@@ -4218,9 +4218,12 @@ def _reflow_param_docstring(docstring, has_first_line=True, width=75):
 docdict["split_naming"] = """
 split_naming : 'neuromag' | 'bids'
     When splitting files, append a filename partition with the appropriate
-    naming schema: for ``'neuromag'``, a split file ``fname.fif`` will be named
-    ``fname.fif``, ``fname-1.fif``, ``fname-2.fif`` etc.; while for ``'bids'``,
-    it will be named ``fname_split-01.fif``, ``fname_split-02.fif``, etc.
+    naming schema. For ``'neuromag'``, a split file ``fname.fif`` will be named
+    ``fname.fif``, ``fname-1.fif``, ``fname-2.fif``, and so on. For ``'bids'``,
+    a filename is expected to consist of parts separated by underscores, like
+    ``<part-1>_<part-N>_<suffix>.fif``, and the according split naming will
+    return filenames like ``<part-1>_<part-N>_split-01_<suffix>.fif``,
+    ``<part-1>_<part-N>_split-02_<suffix>.fif``, and so on.
 """
 
 docdict["src_eltc"] = """