doc: add trace_buf_size description to kernel-parameters.txt

from early boot tracing view, trace_buf_size parameter is important.
it should be documented.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
LKML-Reference: <20090310135200.A48B.A69D9226@jp.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>