Document crash_log.bin and crash_log.bin.tmp log files

http://docs.couchbase.com/admin/admin/Misc/Trbl-logs.html

Please add 2 log files, crash_log.bin and crash_log.bin.tmp

crash_log.bin (and .tmp) is used to pass service crash reports from babysitter to ns_server. I.e. if ns_server is available, any crash of babysitter's child is passed directly to special crash logger service within ns_server. But if this service is not attached to babysiter, then babysitter saves that crash report to disk. So that ns_server can later obtain and log it even if babysitter is restarted.