sysctl: add some hints how to override settings

Also a link to decent documentation for sysrq keys. It is surprising
hard to find.

https://lists.fedoraproject.org/pipermail/devel/2015-February/208412.html

sysctl.d/50-default.conf

@@ -5,9 +5,16 @@
 #  the Free Software Foundation; either version 2.1 of the License, or
 #  (at your option) any later version.
 
-# See sysctl.d(5) and core(5) for for details.
+# See sysctl.d(5) and core(5) for for documentation.
+
+# To override settings in this file, create a local file in /etc
+# (e.g. /etc/sysctl.d/90-override.conf), and put any assignments
+# there.
 
 # System Request functionality of the kernel (SYNC)
+#
+# Use kernel.sysrq = 1 to allow all keys.
+# See http://fedoraproject.org/wiki/QA/Sysrq for a list of values and keys.
 kernel.sysrq = 16
 
 # Append the PID to the core filename