Update docs: Referencing user/channel in a recipe throws error

Looking at the example here where the requirements() method checks if the user and channel are set:
https://docs.conan.io/en/latest/reference/conanfile/attributes.html#user-channel

```
if self.user and self.channel:
# If the recipe is using them, I want to consume my fork.
self.requires("Say/0.1@%s/%s" % (self.user, self.channel))

```
For the scenario where the user/channel are not provided in the `conan create` command I get:
conans.errors.ConanException: user not defined, but self.user is used in conanfile

Is the documentation dated or is the behavior python version-related perhaps?

Python 3.7.1
Conan 1.21.1

Thanks