doc: document --overwrite-ignore

I added this option in git-checkout and git-merge in c1d7036b6b
(checkout,merge: disallow overwriting ignored files with
--no-overwrite-ignore - 2011-11-27) but did not remember to update
documentation. This completes that commit.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -271,6 +271,12 @@ Note that this option uses the no overlay mode by default (see also
 	out anyway. In other words, the ref can be held by more than one
 	worktree.
 
+--overwrite-ignore::
+--no-overwrite-ignore::
+	Silently overwrite ignored files when switching branches. This
+	is the default behavior. Use `--no-overwrite-ignore` to abort
+	the operation when the new branch contains ignored files.
+
 --recurse-submodules::
 --no-recurse-submodules::
 	Using --recurse-submodules will update the content of all initialized

Documentation/git-merge.txt

@@ -87,6 +87,11 @@ will be appended to the specified message.
 	Allow the rerere mechanism to update the index with the
 	result of auto-conflict resolution if possible.
 
+--overwrite-ignore::
+--no-overwrite-ignore::
+	Silently overwrite ignored files from the merge result. This
+	is the default behavior. Use `--no-overwrite-ignore` to abort.
+
 --abort::
 	Abort the current conflict resolution process, and
 	try to reconstruct the pre-merge state.