Previously the --haddock-index=template flag controled both the
template used and whether it's used at all. When no path was set
then it was not used. The problem with that is that since we are
not enabling this feature by default then the default is blank.
That is the default config file would look like:
-- haddock-index:
which doesn't help anyone discover what it means or what a
sensible setting would be. By having a separate toggle to     
enable/disable we can have a default for the index file which
makes it easy to discover in the config file:
-- documentation: False
-- doc-index-file: $datadir/doc/index.html
All the user has to do is uncomment the first line and use True.