sysrq documentation: document why the command header only is shown

Document the interactions between loglevel and the sysrq output.  Also
document how to work round it should output be required on the console.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Cc: Martin Mares <mj@ucw.cz>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/sysrq.txt

@@ -210,6 +210,24 @@ within a function called by handle_sysrq, you must be aware that you are in
 a lock (you are also in an interrupt handler, which means don't sleep!), so
 you must call __handle_sysrq_nolock instead.
 
+*  When I hit a SysRq key combination only the header appears on the console?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Sysrq output is subject to the same console loglevel control as all
+other console output.  This means that if the kernel was booted 'quiet'
+as is common on distro kernels the output may not appear on the actual
+console, even though it will appear in the dmesg buffer, and be accessible
+via the dmesg command and to the consumers of /proc/kmsg.  As a specific
+exception the header line from the sysrq command is passed to all console
+consumers as if the current loglevel was maximum.  If only the header
+is emitted it is almost certain that the kernel loglevel is too low.
+Should you require the output on the console channel then you will need
+to temporarily up the console loglevel using alt-sysrq-8 or:
+
+    echo 8 > /proc/sysrq-trigger
+
+Remember to return the loglevel to normal after triggering the sysrq
+command you are interested in.
+
 *  I have more questions, who can I ask?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 And I'll answer any questions about the registration system you got, also