Documentation: bootconfig: Add a doc for extended boot config

Add a documentation for extended boot config under
admin-guide, since it is including the syntax of boot config.

Link: http://lkml.kernel.org/r/157867230658.17873.9309879174829924324.stgit@devnote2



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