[FIX] clarify MEG empty-room recording naming conventions (#480)

* FIX: clarify empty-room recording naming conventions

* improve wording

Co-authored-by: Richard Höchenberger <richard.hoechenberger@gmail.com>

* Update src/04-modality-specific-files/02-magnetoencephalography.md

Co-authored-by: Richard Höchenberger <richard.hoechenberger@gmail.com>

* add context, reformat paragraphs, add example

* fix list

* improve wording

Co-authored-by: Richard Höchenberger <richard.hoechenberger@gmail.com>

* fix typo

* fix linter

* add metadata and meg/ dir to example

* clarify that metadata sidecars should always be present as well

* limit scope of PR to formatting + clarification

Co-authored-by: Richard Höchenberger <richard.hoechenberger@gmail.com>

src/04-modality-specific-files/02-magnetoencephalography.md

@@ -426,14 +426,24 @@ has to be updated, then for MEG it could be considered to be a new session.
 
 ## Empty-room MEG recordings
 
-Empty-room MEG recordings capture the environment and system noise. Their
-collection is RECOMMENDED, before/during/after each session. This data is stored
-inside a subject folder named `sub-emptyroom`. The `session label` SHOULD be
-that of the date of the empty-room recording (e.g. `ses-YYYYMMDD`). The
-`scans.tsv` file containing the date/time of the acquisition SHOULD also be
-included. Hence, users will be able to retrieve the empty-room recording that
-best matches a particular session with a participant, based on date/time of
-recording.
+Empty-room MEG recordings capture the environmental and recording system's
+noise.
+In the context of BIDS it is RECOMMENDED to perform an empty-room recording for
+each experimental session.
+It is RECOMMENDED to store the empty-room recording inside a subject folder
+named `sub-emptyroom`.
+The label for the `task-<label>` entity in the empty-room recording SHOULD be
+set to `noise`.
+If a `session-<label>` entity is present, its label SHOULD be the date of the
+empty-room recording in the format `YYYYMMDD`, i.e., `ses-YYYYMMDD`.
+The `scans.tsv` file containing the date and time of the acquisition SHOULD
+also be included.
+The rationale is that this naming scheme will allow users to easily retrieve the
+empty-room recording that best matches a particular experimental session, based
+on date and time of the recording.
+It should be possible to query empty-room recordings just like usual subject
+recordings, hence all metadata sidecar files (such as the `channels.tsv`) file
+SHOULD be present as well.
 
 Example:
 
@@ -446,6 +456,5 @@ sub-emptyroom/
         meg/
             sub-emptyroom_ses-20170801_task-noise_meg.ds
             sub-emptyroom_ses-20170801_task-noise_meg.json
+            sub-emptyroom_ses-20170801_task-noise_channels.tsv
 ```
-
-`TaskName` in the `*_meg.json` file should be set to "noise".