docs: bootconfig: Update for mixing value and subkeys

Update document for the mixing value and subkeys on a key.

Link: https://lkml.kernel.org/r/162262196510.264090.15044943496205914148.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>