Explain CI_PROJECT_NAMESPACE better
## What does this MR do?
Explain CI_PROJECT_NAMESPACE better by adding "The project namespace (username or groupname) that is currently being built".
## Are there points in the code the reviewer needs to double check?
Are `username` and `groupname` known words? We use it [here](http://docs.gitlab.com/ee/api/namespaces.html).
## Why was this MR needed?
Because `namespace` isn't a well known term.
## What are the relevant issue numbers?
## Screenshots (if relevant)
See merge request !5407
cbe787c587