Documentation: clarify 'ours' merge strategy

Make it clear in the docs that the merge takes the tree of HEAD and
ignores everything in the other branches.  This should hopefully clear
up confusion, usually caused by the user looking for a strategy that
resolves all conflict hunks in favour of HEAD (which is completely
different and currently not supported).

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/merge-strategies.txt

@@ -29,8 +29,9 @@ octopus::
 	pulling or merging more than one branch.
 
 ours::
-	This resolves any number of heads, but the result of the
-	merge is always the current branch head.  It is meant to
+	This resolves any number of heads, but the resulting tree of the
+	merge is always that of the current branch head, effectively
+	ignoring all changes from all other branches.  It is meant to
 	be used to supersede old development history of side
 	branches.