The single axis is a hyphen separated list of variables.  There is a default configuration (that is also used to show an example).  If the axis variable is not formatted correctly, then the default config is used.

PLATFORM-BROWSER-VERSION #-DEVICE TYPE

I can tighten up that comment as well.  The problem with storing it in a comma seperated list is that jenkins will use that axis variable as the url to access the job info.  However, jenkins reserves commas to mean multiple axis so the resulting page would 404.  I can also use # to seperate values if you prefer