dagop: fix documentation of reachableroots
The previous revset couldn't be correct as it is symmetric in <roots>
and <heads>, but reachableroots has no such symmetry. It makes a
difference with for instance reachableroots(2, 3) where 2 and 3 are
both children of 1.
Differential Revision: https://phab.mercurial-scm.org/D6505
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