Documentation: Add diff.<driver>.* to config

Although the gitattributes page contains comprehensive information
about these configuration options, they should be included in the
config documentation for completeness.

It may be better to rename the "driver" in "diff.<driver>.*" to
something like "content type" or "file type", but for now, let's keep
it consistent across this part of the documentation and the original
description in the gitattributes documentation.

Helped-by: Jakub Narebski <jnareb@gmail.com>
Helped-by: Michael J Gruber <git@drmicha.warpmail.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-config.txt

@@ -60,3 +60,31 @@ diff.tool::
 	`merge.tool` when used by linkgit:git-difftool[1] and has
 	the same valid values as `merge.tool` minus "tortoisemerge"
 	and plus "kompare".
+
+diff.<driver>.command::
+	The custom diff driver command.  See linkgit:gitattributes[5]
+	for details.
+
+diff.<driver>.xfuncname::
+	The regular expression that the diff driver should use to
+	recognize the hunk header.  A built-in pattern may also be used.
+	See linkgit:gitattributes[5] for details.
+
+diff.<driver>.binary::
+	Set this option to true to make the diff driver treat files as
+	binary.  See linkgit:gitattributes[5] for details.
+
+diff.<driver>.textconv::
+	The command that the diff driver should call to generate the
+	text-converted version of a file.  The result of the
+	conversion is used to generate a human-readable diff.  See
+	linkgit:gitattributes[5] for details.
+
+diff.<driver>.wordregex::
+	The regular expression that the diff driver should use to
+	split words in a line.  See linkgit:gitattributes[5] for
+	details.
+
+diff.<driver>.cachetextconv::
+	Set this option to true to make the diff driver cache the text
+	conversion outputs.  See linkgit:gitattributes[5] for details.