Correct the documentation for environment variables

- Unlike the way most configuration systems treat environment variables, 

  they do not overwrite options provided in the configuration files unless 

  the files are specifically written to honour the environment variables.

- For array options the value provided in the environment variable will 

  become the first item (index 0).

  the files are specifically written to honour the environment variables, 

  for instance by using the "+=" operator to append to arrays.

- For array options the value provided will be split on whitespace. 

  Unfortunately at the moment there is no way to escape or quote whitespace 

  within items.  If you find yourself needing to add items with whitespace, 

  consider using configuration files instead, which are interpreted with 

  Bash, or one of the [convenience files](#convenience-files).

Convenience Files