merge: improve clarity of table in update docstring

mercurial/merge.py

@@ -1121,10 +1121,10 def update(repo, node, branchmerge, forc
      n   n    n     y   |    ok     ok     ok
      n   n    y     n   |   merge   (2)    (2)
      n   n    y     y   |   merge   (3)    (3)
-     n   y    *     *   |    ---  discard  ---
-     y   n    y     *   |    ---    (4)    ---
-     y   n    n     *   |    ---    ok     ---
-     y   y    *     *   |    ---    (5)    ---
+     n   y    *     *   |   discard   discard   discard
+     y   n    y     *   |    (4)       (4)      (4)
+     y   n    n     *   |    ok        ok       ok
+     y   y    *     *   |    (5)       (5)      (5)
 
     x = can't happen
     * = don't-care