asdf.sh: comments for script path determination
asdf.sh: comments for script path determination

Update `asdf.sh` to explain that `$_`, `${BASH_SOURCE[0]}`, and `$0`
are used by Korn, Bash, and Zsh (and others) to obtain the path to the
script, and what those special variables mean.

Stratus3D/asdf,asdf-vm/asdf