merge-strategies: avoid implying that "-s theirs" exists

The description of `-Xours` merge option has a parenthetical note
that tells the readers that it is very different from `-s ours`,
which is correct, but the description of `-Xtheirs` that follows it
carelessly says "this is the opposite of `ours`", giving a false
impression that the readers also need to be warned that it is very
different from `-s theirs`, which in reality does not even exist.

Clarify it a bit to avoid misleading readers.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/merge-strategies.txt

@@ -39,7 +39,8 @@ even look at what the other tree contains at all.  It discards everything
 the other tree did, declaring 'our' history contains all that happened in it.
 
 theirs;;
-	This is the opposite of 'ours'.
+	This is the opposite of 'ours'; note that, unlike 'ours', there is
+	no 'theirs' merge stragegy to confuse this merge option with.
 
 patience;;
 	With this option, 'merge-recursive' spends a little extra time