Description of problem:
The URL of a content unit depends both on particular distributor configuration 
applied via the POST API call as well as on the related Apache configuration.
For a yum_distributor, these two parts are
 - the "config.relative_url" field in the creating POST API call
 - the Alias directive in the Pulp-rpm-plugin Apache config-file[1],
   set by default to: /pulp/repos/

This should be mentioned in the docs[2].

[1] /etc/httpd/conf.d/pulp_rpm.conf 
[2] https://pulp-dev-guide.readthedocs.org/en/pulp-2.2/integration/rest-api/repo/cud.html#associate-a-distributor-with-a-repository