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