Add tags from the Documentation project

Thanks to psynaptic for pointing me in the right direction.

The API docs for most (all?) D6 hooks are generated from the Documentation project. I think there are some useful D7 tags in there, too.

We should run ctags on the Documentation project as well as Drupal core to generate our tags files. This will allow auto-completion of hook names in D6 and to-be-determined benefits in D7. ;) Also, if the user has the Documentation project installed, then (for example, using D7) :tag theme_block should go to the lines

/**
 * Implemented using the block.tpl.php template.
 *
 * @see block.tpl.php
 */
function theme_block($variables) {
  // This function is never used; see the corresponding template file instead.
}

in documentation/developer/theme.php. Then one can follow the tag for block.tpl.php to get to modules/block/block.tpl.php.

When we get around to improving doxygen syntax highlighting, we can make @see block.tpl.php look like a link.

See the recipe in #1347302-1: Tags files are too big. and #4 for how we generate tags files.

We should also add a configuration option so the user can specify where to look for the Documentation project in case it is not under DRUPAL_ROOT.