git-apply.txt: mention the behavior inside a subdir

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

Documentation/git-apply.txt

@@ -21,6 +21,8 @@ SYNOPSIS
 DESCRIPTION
 -----------
 Reads the supplied diff output (i.e. "a patch") and applies it to files.
+When running from a subdirectory in a repository, patched paths
+outside the directory are ignored.
 With the `--index` option the patch is also applied to the index, and
 with the `--cached` option the patch is only applied to the index.
 Without these options, the command applies the patch only to files,