printk/index: Printk index feature documentation

Document the printk index feature. The primary motivation is to
explain that it is not creating KABI from particular printk() calls.

Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Chris Down <chris@chrisdown.name>
Signed-off-by: Petr Mladek <pmladek@suse.com>