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/