Documentation/git-merge-tree.txt: document -X

Add an entry in the 'merge-tree' builtin documentation for
-X/--strategy-option (added in 6a4c9e7 (merge-tree: add -X strategy
option, 2023-09-24)). The same option is documented for 'merge', 'rebase',
'revert', etc. in their respective Documentation/ files, so let's do the
same for 'merge-tree'.

Signed-off-by: Victoria Dye <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: vdye

Documentation/git-merge-tree.txt

@@ -72,6 +72,11 @@ OPTIONS
 As the merge-base is provided directly, <branch1> and <branch2> do not need
 to specify commits; trees are enough.
 
+-X<option>::
+--strategy-option=<option>::
+	Pass the merge strategy-specific option through to the merge strategy.
+	See linkgit:git-merge[1] for details.
+
 [[OUTPUT]]
 OUTPUT
 ------