How can I get documentation on a variable listed in the Features UI provided by Strongarm? When looking through the list of variables provided by Strongarm, there are several that sound like they might be useful to include but what they actually refer to is impossible to guess from the name alone (e.g., mail_system).  How can I quickly look up unknown variables to see what it is that they do?
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.