Add min_input_length to SQLA example and docs.

examples/sqla/app.py

@@ -130,8 +130,11 @@ class PostAdmin(sqla.ModelView):
             'fields': (User.username, User.email)
         },
         'tags': {
-            'fields': (Tag.name,)
-        }
+            'fields': (Tag.name,),
+            'minimum_input_length': 0,
+            'placeholder': 'Please select',
+            'page_size': 5,
+        },
     }
 
     def __init__(self, session):

flask_admin/model/base.py

@@ -671,7 +671,9 @@ class BaseModelView(BaseView, ActionsMixin):
                 form_ajax_refs = {
                     'user': {
                         'fields': ('first_name', 'last_name', 'email'),
-                        'page_size': 10
+                        'placeholder': 'Please select',
+                        'page_size': 10,
+                        'minimum_input_length': 0,
                     }
                 }