Self-document the XDG_STATE_HOME fallback

[jasonkarns]

Followup documentation/comments for #1146

Document XDG_STATE_HOME fallback value

The RUBY_DEBUG_HISTORY_FILE has a default value of $XDG_STATE_HOME/rdbg/history which is already documented. However, the fallback value for the env variable XDG_STATE_HOME is not documented. Users who are unfamiliar with XDG basedir spec may not know that the default value for it is ~/.local/state/.

This change uses bash parameter expansion syntax to "self-document" the fallback value for the variable when it is unset.

Note: I used the - default syntax, not the :- syntax. This is because the current ruby code does not check if XDG_STATE_HOME is empty, it merely checks if it is set. So an empty-string value would not trigger the fallback location to be used.

This PR was spawned from the following discussion: #1146 (comment)

[ko1]

ok thank you!

it is another idea that adding a (sub)section "Files" and describe related files.