git-fast-import.txt: improve documentation for quoted paths

The documentation mentioned only newlines and double quotes as
characters needing escaping, but the backslash also needs it. Also, the
documentation was not clearly saying that double quotes around the file
name were required (double quotes in the examples could be interpreted as
part of the sentence, not part of the actual string).

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -562,8 +562,12 @@ A `<path>` string must use UNIX-style directory separators (forward
 slash `/`), may contain any byte other than `LF`, and must not
 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"`.
+A path can use C-style string quoting; this is accepted in all cases
+and mandatory if the filename starts with double quote or contains
+`LF`. In C-style quoting, the complete name should be surrounded with
+double quotes, and any `LF`, backslash, or double quote characters
+must be escaped by preceding them with a backslash (e.g.,
+`"path/with\n, \\ and \" in it"`).
 
 The value of `<path>` must be in canonical form. That is it must not: