docs: improve docstrings in __init__.py and schema.py

src/flask_marshmallow/__init__.py

@@ -57,7 +57,7 @@
 
 
 def _attach_fields(obj):
-    """Attach all the marshmallow fields classes to ``obj``, including
+    """Attach all the marshmallow field classes to ``obj``, including
     Flask-Marshmallow's custom fields.
     """
     for attr in base_fields.__all__:
@@ -81,7 +81,7 @@ class Marshmallow:
     all fields in :mod:`marshmallow.fields`, as well as the Flask-specific
     fields in :mod:`flask_marshmallow.fields`.
 
-    You can declare schema like so::
+    You can declare a schema like so::
 
         class BookSchema(ma.Schema):
             id = ma.Integer(dump_only=True)

src/flask_marshmallow/schema.py

@@ -25,6 +25,7 @@ def jsonify(
         :param bool many: Whether `obj` should be serialized as an instance
             or as a collection. If None, defaults to the value of the
             `many` attribute on this Schema.
+        :param args: Additional positional arguments passed to `flask.jsonify`.
         :param kwargs: Additional keyword arguments passed to `flask.jsonify`.
 
         .. versionchanged:: 0.6.0