Tag linked_structure for single-crystal experiments

Author: AndrewSazonov

docs/docs/user-guide/parameters/experiment/linked_structure.md

@@ -6,22 +6,27 @@ title: linked_structure
 
 [pd-neut-cwl][3]{:.label-experiment}
 [pd-neut-tof][3]{:.label-experiment} [pd-xray][3]{:.label-experiment}
+[sc-neut-cwl][3]{:.label-experiment}
 
 ## :material-identifier: structure_id { #linked-structure-structure-id }
 
 | Access                               | Source                  |
 | ------------------------------------ | ----------------------- |
 | linked_structures['ID'].structure_id | [code][0]{:.label-cif}  |
+| linked_structure.structure_id        | [code][0]{:.label-cif}  |
 | \_linked_structure.structure_id      | [Edifa][0]{:.label-cif} |
 | \_pd_phase_block.id                  | [pdCIF][0]{:.label-cif} |
 
-Identifier of the linked structure.
+Identifier of the linked structure. The plural `linked_structures['ID']`
+form is used for powder experiments (one or more linked phases); the
+scalar `linked_structure` form is used for single-crystal experiments.
 
 ## :material-scale: scale { #linked-structure-scale }
 
 | Access                        | Source                  |
 | ----------------------------- | ----------------------- |
 | linked_structures['ID'].scale | [code][0]{:.label-cif}  |
+| linked_structure.scale        | [code][0]{:.label-cif}  |
 | \_linked_structure.scale      | [Edifa][0]{:.label-cif} |
 | \_pd_phase_block.scale        | [pdCIF][0]{:.label-cif} |