revlog: fix revset in reachableroots docstring

`reachableroots` will only return a subset of `roots` when `includepath` is
False. For example, given the following linear DAG:

  2
  |
  1
  |
  0

Using roots=0+2, heads=1, the definition in the docstring does not match what
`reachableroots` actually does:

  ipdb> repo.changelog.reachableroots(0, roots=[0,2],heads=[1])
  [0]
  ipdb> repo.revs('heads(::(0+2) & (0+2)::1)')
  <baseset+ [1]>

The fix is to do `heads & ::roots` (or `heads & heads::roots`) first, then
select their ancestors:

  ipdb> repo.revs('heads(::((0+2) & (0+2)::1))')
  <baseset+ [0]>

The docstring was introduced by fd92bfbbe02d9 (2015-06-19 "revset: rename
revsbetween to reachableroots and add an argument"), which introduced the
`includepath=False` behavior for graphlog grandparents use-case. I believe
the docstring instead of the code should be changed because changing the
code to match the docstring can result in suboptimal graphlog like:

  o
  :\
  : o
  : :
  :/
  o

As opposite to the current "linearized" graphlog:

  o
  |
  o
  :
  o

Differential Revision: https://phab.mercurial-scm.org/D7518

mercurial/revlog.py

@@ -1279,7 +1279,7 b' class revlog(object):'
         return bool(self.reachableroots(a, [b], [a], includepath=False))
 
     def reachableroots(self, minroot, heads, roots, includepath=False):
-        """return (heads(::<roots> and <roots>::<heads>))
+        """return (heads(::(<roots> and <roots>::<heads>)))
 
         If includepath is True, return (<roots>::<heads>)."""
         try: