diff: describe behavior by using --from/--to instead of varying revision count
I very recently updated the documentation to prefer --from/--to
over -r, but I missed the plain-text description of how the command
behaves when given different numbers of revisions (I guess I just
scanned the text for "-r"). This patch fixes that.
Differential Revision: https://phab.mercurial-scm.org/D9562