Very less information I found on xp_readerrorlog
Exec sp_helpextendedproc 'xp_ReadErrorLog' gives "xpstar.dll" as source.
there is no page even on MSDN Library
Here is the only usefull link I found.
https://sqlserver-help.com/2014/12/10/sql-internals-useful-parameters-for-xp_readerrorlog/
ThanksSaurabh.D