Clarify the special branch 'master'

The 'master' branch name is confusing as it indicates some special branch name, where it is actually just a 'ref'. 'branches' is a special name as it indicates 'all' branches.

Lets clarify the example a little to indicate this.

TODO: Show why the user why the use of 'branches' is needed at all, as what does all branches mean? What is the opposite of all branches, no branches? So the job runs never? 'master' is just a branch like any other. And what makes master different from the 'default' branch (which is very often 'master').