revset: improve documentation about ordering handling The old documentation is a bit confusing. Namely, it's unclear whether `define` means "I should ALWAYS define a new order", or "I should SOMETIMES define a new order", and if it's the latter, what's the difference between `define` and `any`? This patch clarifies that and adds more examples. Differential Revision: https://phab.mercurial-scm.org/D523