Cleanup: add some deformation-related comments

Add documentation for `BKE_id_defgroup_list_get()` and document that
`CD_MDEFORMVERT` mesh layers contain `MDeformVert` structs.

No functional changes.

source/blender/blenkernel/BKE_deform.h

@@ -37,6 +37,11 @@ int BKE_object_defgroup_active_index_get(const struct Object *ob);
  */
 void BKE_object_defgroup_active_index_set(struct Object *ob, int new_index);
 
+/**
+ * Return the ID's vertex group names.
+ * Supports Mesh (ME), Lattice (LT), and GreasePencil (GD) IDs.
+ * \return ListBase of bDeformGroup pointers.
+ */
 const struct ListBase *BKE_id_defgroup_list_get(const struct ID *id);
 struct ListBase *BKE_id_defgroup_list_get_mutable(struct ID *id);
 int BKE_id_defgroup_name_index(const struct ID *id, const char *name);

source/blender/makesdna/DNA_customdata_types.h

@@ -92,7 +92,7 @@ typedef enum eCustomDataType {
 #ifdef DNA_DEPRECATED_ALLOW
   CD_MSTICKY = 1, /* DEPRECATED */
 #endif
-  CD_MDEFORMVERT = 2,
+  CD_MDEFORMVERT = 2, /* Array of `MDeformVert`. */
   CD_MEDGE = 3,
   CD_MFACE = 4,
   CD_MTFACE = 5,