docs: sysctl/kernel: document ftrace entries

Based on the ftrace documentation, the tp_printk boot parameter
documentation, and the implementation in kernel/trace/trace.c.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20200429205757.8677-1-steve@sk2.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>