git-clean: add documentation for interactive git-clean

Add new section "Interactive mode" for documentation of interactive
git-clean.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clean.txt

@@ -39,8 +39,8 @@ OPTIONS
 
 -i::
 --interactive::
-	Show what would be done and the user must confirm before actually
-	cleaning.
+	Show what would be done and clean files interactively. See
+	``Interactive mode'' for details.
 
 -n::
 --dry-run::
@@ -69,6 +69,67 @@ OPTIONS
 	Remove only files ignored by Git.  This may be useful to rebuild
 	everything from scratch, but keep manually created files.
 
+Interactive mode
+----------------
+When the command enters the interactive mode, it shows the
+files and directories to be cleaned, and goes into its
+interactive command loop.
+
+The command loop shows the list of subcommands available, and
+gives a prompt "What now> ".  In general, when the prompt ends
+with a single '>', you can pick only one of the choices given
+and type return, like this:
+
+------------
+    *** Commands ***
+	1: clean                2: filter by pattern    3: select by numbers
+	4: ask each             5: quit                 6: help
+    What now> 1
+------------
+
+You also could say `c` or `clean` above as long as the choice is unique.
+
+The main command loop has 6 subcommands.
+
+clean::
+
+   Start cleaning files and directories, and then quit.
+
+filter by pattern::
+
+   This shows the files and directories to be deleted and issues an
+   "Input ignore patterns>>" prompt. You can input space-seperated
+   patterns to exclude files and directories from deletion.
+   E.g. "*.c *.h" will excludes files end with ".c" and ".h" from
+   deletion. When you are satisfied with the filtered result, press
+   ENTER (empty) back to the main menu.
+
+select by numbers::
+
+   This shows the files and directories to be deleted and issues an
+   "Select items to delete>>" prompt. When the prompt ends with double
+   '>>' like this, you can make more than one selection, concatenated
+   with whitespace or comma.  Also you can say ranges.  E.g. "2-5 7,9"
+   to choose 2,3,4,5,7,9 from the list.  If the second number in a
+   range is omitted, all remaining patches are taken.  E.g. "7-" to
+   choose 7,8,9 from the list.  You can say '*' to choose everything.
+   Also when you are satisfied with the filtered result, press ENTER
+   (empty) back to the main menu.
+
+ask each::
+
+  This will start to clean, and you must confirm one by one in order
+  to delete items. Please note that this action is not as efficient
+  as the above two actions.
+
+quit::
+
+  This lets you quit without do cleaning.
+
+help::
+
+  Show brief usage of interactive git-clean.
+
 SEE ALSO
 --------
 linkgit:gitignore[5]