docs: Improve wording of env vars section

[hyperupcall]

Improves wording of environmental variables section in two ways:

1. No longer refer to ~/.asdf as an absolute path

Beginners may take this too literally and do something like export ASDF_DATA_DIR=~/.asdf, which won't work since tilda expansion doesn't occur here (since the ~/ isn't the first two characters in the word, the AS in ASDF_DATA_DIR is).

To prevent misconceptions or potential issues, this has been removed.

2. Update description of ASDF_DIR

This variable no longer defaults to ~/.asdf, but instead is dynamically set and always refers to the true location of the source files.

[jthegedus]

@hyperupcall I proposed a larger rewrite of the whole section. I used some of your wording throughout and removed the tilde usage. Have a look and let me know what you think.

[hyperupcall]

That's a nice improvement! Once I finally finish up #1351, the extra space will make it easier to write and read the new behavior.

I did change the wording of ASDF_DATA_DIR to be more accurate - since $HOME is always set, ASDF_DATA_DIR will default to $HOME/.asdf. Which is good because otherwise asdf would require superuser priviledges if installed through the AUR.