revset: remove part of the commonancestors() comment The reason is that: - it shows up in "hg help revset", but it doesn't look like documentation targeted at users - it doesn't make sense to me: it doesn't say what happens with < 2 revisions, and is not quite right because my understanding is that this revset was created precisely because "::x and ::y" was not quite the same (when x and y don't evaluate to singletons). Differential Revision: https://phab.mercurial-scm.org/D4739