* Write a description to outline the scope and mechanism of runlabel.
* Describe the variables/attributes that we want to be used.
* Do not describe the --optN or OPTN flags/variables since they are
already hidden flags and date back to the Atomic days.
* Update references to other man pages.
* Remove unsupported variables (e.g., SUDO_*) which caused confusion.
Fixes: #10799
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>