How can I get documentation on a variable listed in the Features UI provided by Strongarm?
Unfortunately, this is a shortcoming of the Variables system in Drupal: there is no centralized place documenting what they do. This is one reason why it is recommended that modules implementing their own advanced configuration (more than a few key-value pairs) do so using their own tables in hook_schema(), where everything can be documented. That having been said, you can grep around for where they're set/read from your Drupal root:

grep -rn 'variable_[gs]et(' . | grep 'foo_variable'


By checking the documentation for the functions in which they're set, you can typically glean their purpose.