Documentation: git diff --check respects core.whitespace

Fix documentation on "git diff --check" by adopting the description from
"git apply --whitespace".

Signed-off-by: Christof Krüger <git@christof-krueger.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -194,10 +194,14 @@ endif::git-format-patch[]
 
 ifndef::git-format-patch[]
 --check::
-	Warn if changes introduce trailing whitespace
-	or an indent that uses a space before a tab. Exits with
-	non-zero status if problems are found. Not compatible with
-	--exit-code.
+	Warn if changes introduce whitespace errors.  What are
+	considered whitespace errors is controlled by `core.whitespace`
+	configuration.  By default, trailing whitespaces (including
+	lines that solely consist of whitespaces) and a space character
+	that is immediately followed by a tab character inside the
+	initial indent of the line are considered whitespace errors.
+	Exits with non-zero status if problems are found. Not compatible
+	with --exit-code.
 endif::git-format-patch[]
 
 --full-index::