Documentation/fast-import: put explanation of M 040000 <dataref> "" in context

Omit needless words ("Additionally ... <path> may also" is redundant).
While at it, place the explanation of this special case after the
general rules for paths to provide the reader with some context.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -534,9 +534,6 @@ start with double quote (`"`).
 If an `LF` or double quote must be encoded into `<path>` shell-style
 quoting should be used, e.g. `"path/with\n and \" in it"`.
 
-Additionally, in `040000` mode, `<path>` may also be an empty string
-(`""`) to specify the root of the tree.
-
 The value of `<path>` must be in canonical form. That is it must not:
 
 * contain an empty directory component (e.g. `foo//bar` is invalid),
@@ -545,6 +542,8 @@ The value of `<path>` must be in canonical form. That is it must not:
 * contain the special component `.` or `..` (e.g. `foo/./bar` and
   `foo/../bar` are invalid).
 
+The root of the tree can be represented by an empty string as `<path>`.
+
 It is recommended that `<path>` always be encoded using UTF-8.
 
 `filedelete`