Document graphql_type on @strawberry.field

This documents a use case where the GraphQL type is not the same as data
being used. This is useful if you are using an ORM for your data and
building an API layer against it.

docs/general/queries.md

@@ -55,6 +55,30 @@ class Query:
         return "Strawberry"
 ```
 
+The decorator syntax supports specifying a `graphql_type` for cases when the
+return type of the function does not match the GraphQL type:
+
+```python
+class User:
+    id: str
+    name: str
+
+    def __init__(self, id: str, name: str):
+        self.id = id
+        self.name = name
+
+@strawberry.type(name="User")
+class UserType:
+    id: strawberry.ID
+    name: str
+
+@strawberry.type
+class Query:
+    @strawberry.field(graphql_type=UserType)
+    def user(self) -> User
+        return User(id="ringo", name="Ringo")
+```
+
 ## Arguments
 
 GraphQL fields can accept arguments, usually to filter out or retrieve specific