docs: add missing period and capitalize where appropriate (#1653)

docs/usage/models.md

@@ -37,20 +37,20 @@ if no `ValidationError` is raised, you know the resulting model instance is vali
 ```py
 assert user.id == 123
 ```
-fields of a model can be accessed as normal attributes of the user object
-the string '123' has been cast to an int as per the field type
+Fields of a model can be accessed as normal attributes of the user object.
+The string '123' has been cast to an int as per the field type
 ```py
 assert user.name == 'Jane Doe'
 ```
-name wasn't set when user was initialised, so it has the default value
+`name` wasn't set when user was initialised, so it has the default value
 ```py
 assert user.__fields_set__ == {'id'}
 ```
-the fields which were supplied when user was initialised:
+The fields which were supplied when user was initialised:
 ```py
 assert user.dict() == dict(user) == {'id': 123, 'name': 'Jane Doe'}
 ```
-either `.dict()` or `dict(user)` will provide a dict of fields, but `.dict()` can take numerous other arguments.
+Either `.dict()` or `dict(user)` will provide a dict of fields, but `.dict()` can take numerous other arguments.
 ```py
 user.id = 321
 assert user.id == 321