Mention register_error_handler in errorhandler doc

The documentation for @errorhandler gives "app.error_handler_spec[None][404] = page_not_found" as an example for adding an error handler without the decorator.  However, register_error_handler appears to be the correct way to do this (added 0.7), and it eliminates the problems with modifying error_handler_spec directly.
Merge pull request #707 from xsleonard/patch-1

Mention register_error_handler in errorhandler doc