help: avoid using "$n" parameter in revsetalias example Because parsing "$n" requires a crafted tokenizer, it exists only for backward compatibility (as documented in revset._tokenizealias.) This patch updates the examples so that users are encouraged to use symbolic names instead of "$n"s. I'm going to implement alias expansion in templater, which won't support "$n" parameters to make my life easier. Templater is more complicated than revset because tokenizer and parser call each other.