bug#29044: sort --debug results improvement

[Dan Jacobson]

$ sort -k 2n -k 3n --debug file.txt
sort: using simple byte comparison
sort: key 1 is numeric and spans multiple fields
sort: key 2 is numeric and spans multiple fields
41 011 92.3 亞太
   ___
       ____
________________
41 011 97.1 大漢
   ___
       ____

OK but they look like they only span one field.

Also the user is confused if
________________
is a "key 3", or just a separator.

Therefore please say
": key 1" or "1" etc. at the end of each of them.
This is also important if there many keys.

And add a separator bar, made of -, =, etc. but not _.

Also the Info documentation doesn't mention how to inflence
"sort: using simple byte comparison"
which seems to always be printed when using --debug no matter what.