linux/Config.in: clarify longterm 2.6 URL exception

As this question has popped up multiple times on the mailing list, clarify
that selecting longterm 2.6 kernels requires the 'custom tarball' option
instead of 'custom version'.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

linux/Config.in

@@ -40,6 +40,11 @@ config BR2_LINUX_KERNEL_CUSTOM_VERSION
 	  This option allows to use a specific official version from
 	  kernel.org, like 2.6.x, 2.6.x.y, 3.x.y, ...
 
+	  Note: you cannot use this option to select a _longterm_ 2.6
+	  kernel, because these kernels are not located at the standard
+	  URL at kernel.org. Instead, select "Custom tarball" and
+	  specify the right URL directly.
+
 config BR2_LINUX_KERNEL_CUSTOM_TARBALL
 	bool "Custom tarball"
 	help