Merge branch 'metadata-type-docs' into 'master'

Add metatype docs

See merge request !680

arkindex/documents/api/elements.py

@@ -765,7 +765,18 @@ class RegionBulkCreate(CreateAPIView, UpdateAPIView):
 
 class ElementMetadata(CreateAPIView):
     """
-    Create a medadata on an existing element
+    Create a metadata on an existing element.
+
+    The `date` type allows dates to be parsed and indexed for search APIs. The supported date formats are as follows:
+
+    * `YYYY`: `1995` (recommended)
+    * `YYYY-MM`: `1995-02` (recommended)
+    * `YYYY-MM-DD`: `1995-02-16` (recommended)
+    * `/YYYY(.*)month/`: `1995, february`, with months in French or English (case-insensitive)
+    * `YYYY-YYYY`: `1995-1996` (yields a date interval)
+
+    The `reference` type is also indexed in search APIs and allows searching by
+    known identifiers or references, or any arbitrary string.
     """
     permission_classes = (IsVerified, )
     serializer_class = MetaDataUpdateSerializer