git-fast-import.txt: fix wording about where ls command can appear

The docs claimed `ls` commands could appear almost anywhere, but the
code told a different story.  Modify the docs to match the code.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -1016,8 +1016,8 @@ printing a blob from the active commit (with `cat-blob`) or copying a
 blob or tree from a previous commit for use in the current one (with
 `filemodify`).
 
-The `ls` command can be used anywhere in the stream that comments are
-accepted, including the middle of a commit.
+The `ls` command can also be used where a `filemodify` directive can
+appear, allowing it to be used in the middle of a commit.
 
 Reading from the active commit::
 	This form can only be used in the middle of a `commit`.