Document drupal_alter() API change

It's not clear to me that this API change is viable. Currently spaces implements `hook_spaces_dashboard_block_access_alter()` on behalf of views and needs to reliably be able to do a check against the access plugin of a view. With the old API it has direct access to the block->delta and (could lookup the hash and) load the view. Now we'll need to split the bid on '-' first.

My question here is how reliable that is. Is there code anywhere that will prevent dashes from turning up in the module name or delta? If not we'll probably need a more complex data structure to pass through the alter.