Documentation: document pitfalls with 3-way merge

Oftentimes people will make the same change in two branches, revert the change
in one branch, and then be surprised when a merge reinstitutes that change when
the branches are merged.  Add an explanatory paragraph that explains that this
occurs and the reason why, so people are not surprised.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/merge-strategies.txt

@@ -113,3 +113,11 @@ subtree::
 	match the tree structure of A, instead of reading the trees at
 	the same level. This adjustment is also done to the common
 	ancestor tree.
+
+With the strategies that use 3-way merge (including the default, 'recursive'),
+if a change is made on both branches, but later reverted on one of the
+branches, that change will be present in the merged result; some people find
+this behavior confusing.  It occurs because only the heads and the merge base
+are considered when performing a merge, not the individual commits.  The merge
+algorithm therefore considers the reverted change as no change at all, and
+substitutes the changed version instead.