doc/edges: mention the edge-field option in examples

doc/md/schema-edges.md

@@ -422,6 +422,28 @@ func Do(ctx context.Context, client *ent.Client) error {
 }
 ```
 
+Note that, the foreign-key column can be configured and exposed as an entity field using the
+[Edge Field](#edge-field) option as follows:
+
+```go {4,14}
+// Fields of the User.
+func (User) Fields() []ent.Field {
+	return []ent.Field{
+		field.Int("spouse_id").
+			Optional(),
+    }
+}
+
+// Edges of the User.
+func (User) Edges() []ent.Edge {
+	return []ent.Edge{
+		edge.To("spouse", User.Type).
+			Unique().
+			Field("spouse_id"),
+	}
+}
+```
+
 The full example exists in [GitHub](https://github.com/ent/ent/tree/master/examples/o2obidi).
 
 ## O2M Two Types
@@ -503,6 +525,30 @@ func Do(ctx context.Context, client *ent.Client) error {
 	return nil
 }
 ```
+
+Note that, the foreign-key column can be configured and exposed as an entity field using the 
+[Edge Field](#edge-field) option as follows:
+
+```go {4,15}
+// Fields of the Pet.
+func (Pet) Fields() []ent.Field {
+	return []ent.Field{
+		field.Int("owner_id").
+			Optional(),
+    }
+}
+
+// Edges of the Pet.
+func (Pet) Edges() []ent.Edge {
+	return []ent.Edge{
+		edge.From("owner", User.Type).
+			Ref("pets").
+			Unique().
+			Field("owner_id"),
+	}
+}
+```
+
 The full example exists in [GitHub](https://github.com/ent/ent/tree/master/examples/o2m2types).
 
 ## O2M Same Type
@@ -612,6 +658,29 @@ func Do(ctx context.Context, client *ent.Client) error {
 }
 ```
 
+Note that, the foreign-key column can be configured and exposed as an entity field using the
+[Edge Field](#edge-field) option as follows:
+
+```go {4,15}
+// Fields of the Node.
+func (Node) Fields() []ent.Field {
+	return []ent.Field{
+		field.Int("parent_id").
+			Optional(),
+    }
+}
+
+// Edges of the Node.
+func (Node) Edges() []ent.Edge {
+	return []ent.Edge{
+		edge.To("children", Node.Type).
+			From("parent").
+			Unique().
+			Field("parent_id"),
+	}
+}
+```
+
 The full example exists in [GitHub](https://github.com/ent/ent/tree/master/examples/o2mrecur).
 
 ## M2M Two Types

doc/website/src/css/custom.css

@@ -819,4 +819,11 @@ html[data-theme='dark'] .header-slack-link:before {
 
 html[data-theme='dark'] .navbar__logo {
   filter: invert(100%);
+}
+
+.docusaurus-highlight-code-line {
+  background-color: rgb(72, 77, 91);
+  display: block;
+  margin: 0 calc(-1 * var(--ifm-pre-padding));
+  padding: 0 var(--ifm-pre-padding);
 }