finish fields docs intial ver,add test for related name, fix child_name(s) in reverse relations

orm/fields/base.py

@@ -35,12 +35,12 @@ class BaseField:
     @property
     def is_required(self) -> bool:
         return (
-            not self.nullable and not self.has_default and not self.is_auto_primary_key
+                not self.nullable and not self.has_default and not self.is_auto_primary_key
         )
 
     @property
     def default_value(self) -> Any:
-        default = self.default if self.default is not None else self.server_default
+        default = self.default
         return default() if callable(default) else default
 
     @property