fix: avoid using an atlas name for a segmentation label

As @CPernet mentioned, the spec was unclear because the example illustrating
``seg-`` was using an atlas name for the label.

I grepped the whole spec for ``seg-`` and only found ``seg-Desikan``.

This commit replaces that label, edits a bit the text around it and
updates the `seg-` entity definition in the schema to be more clear.

Even though its use with ``atlas-`` was already mentioned:

> For atlases, `seg-<label>` distinguish different realizations of a given
> segmentation or parcellation.
> For example, the [Yeo 2011 atlas](https://doi.org/10.1152/jn.00338.2011)
> is distributed within *FreeSurfer* with two different parcellations
> (*7 networks* and *17 networks*).

I have updated the entity's description to be more clear (hopefully).

src/derivatives/imaging.md

@@ -293,9 +293,11 @@ In this case, the mask suffix MUST be used,
 the [`label` entity](../appendices/entities.md#label)) SHOULD be used
 to specify the masked structure
 (see [Common image-derived labels](#common-image-derived-labels)),
-and the [`seg` entity](../appendices/entities.md#segmentation) SHOULD be defined.
+and the [`seg` entity](../appendices/entities.md#segmentation) specifying
+what segmentation generated the mask SHOULD be defined.
 
-For example:
+For example, we can specify the gray-matter mask corresponding to the
+results of a brain tissue segmentation algorithm as:
 
 <!-- This block generates a file tree.
 A guide for using macros can be found at
@@ -306,8 +308,8 @@ A guide for using macros can be found at
     "pipeline": {
         "sub-001": {
             "anat": {
-                "sub-001_space-orig_seg-Desikan_label-GM_mask.nii.gz": "",
-                "sub-001_space-orig_seg-Desikan_label-GM_mask.json": "",
+                "sub-001_space-orig_seg-tissue_label-GM_mask.nii.gz": "",
+                "sub-001_space-orig_seg-tissue_label-GM_mask.json": "",
                 },
             },
         }