Documentation: add ARM user_debug to kernel-parameters.txt

Usually kernel parameters are documented in kernel-parameters.txt
but user_debug is only documented in the Kconfig. Document the
option and point to the Kconfig help text for more info.
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: NRandy Dunlap <rdunlap@xenotime.net>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/kernel-parameters.txt

@@ -2635,6 +2635,16 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 					medium is write-protected).
 			Example: quirks=0419:aaf5:rl,0421:0433:rc
 
+	user_debug=	[KNL,ARM]
+			Format: <int>
+			See arch/arm/Kconfig.debug help text.
+				 1 - undefined instruction events
+				 2 - system calls
+				 4 - invalid data aborts
+				 8 - SIGSEGV faults
+				16 - SIGBUS faults
+			Example: user_debug=31
+
 	userpte=
 			[X86] Flags controlling user PTE allocations.