Fixed #23281 -- Added "concrete model" to glossary.

Thanks knowledgepoint-devs for the suggestion.

docs/glossary.txt

@@ -6,6 +6,10 @@ Glossary
 
 .. glossary::
 
+    concrete model
+        A non-abstract (:attr:`abstract=False
+        <django.db.models.Options.abstract>`) model.
+
     field
         An attribute on a :term:`model`; a given field usually maps directly to
         a single database column.

docs/ref/models/fields.txt

@@ -1522,7 +1522,7 @@ accepted by :class:`ForeignKey`, plus one extra argument:
 .. attribute:: OneToOneField.parent_link
 
     When ``True`` and used in a model which inherits from another
-    (concrete) model, indicates that this field should be used as the
+    :term:`concrete model`, indicates that this field should be used as the
     link back to the parent class, rather than the extra
     ``OneToOneField`` which would normally be implicitly created by
     subclassing.