Explain in mke2fs and tune2fs man pages that shared journals are not supported

Addresses Debian Bug: #316040

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/misc/tune2fs.8.in b/misc/tune2fs.8.in
index 47b247e..ef53009 100644
--- a/misc/tune2fs.8.in
+++ b/misc/tune2fs.8.in
@@ -242,6 +242,11 @@
 @JDEV@.I external-journal
 @JDEV@must be formatted with the same block
 @JDEV@size as filesystems which will be using it.
+@JDEV@In addition, while there is support for attaching
+@JDEV@multiple filesystems to a single external journal,
+@JDEV@the Linux kernel and 
+@JDEV@.BR e2fsck (8)
+@JDEV@do not currently support shared external journals yet.
 @JDEV@.IP
 @JDEV@Instead of specifying a device name directly,
 @JDEV@.I external-journal