docs: document autostring

Author: iloveitaly

sqlmodel/sql/sqltypes.py

@@ -5,6 +5,12 @@
 
 
 class AutoString(types.TypeDecorator):  # type: ignore
+    """
+    Determines the best sqlalchemy string type based on the database dialect.
+
+    For example, when using Postgres this will return sqlalchemy's String()
+    """
+
     impl = types.String
     cache_ok = True
     mysql_default_length = 255