Add a comment about where PHP error_log() output goes. #2680

[jefft]

Summary

Per #2680, this adds a comment to the php plugin's devbox.d/php/php-fpm.conf explaining where to find error_log() output for PHP scripts running under PHP-FPM - because despite appearances, they do not go to the error_log file configured at the top:

[global]
pid = ${PHPFPM_PID_FILE}
error_log = ${PHPFPM_ERROR_LOG_FILE}
daemonize = yes

It is a verbose comment, and arguably it's not our job to be educating numpties, but it would have helped me. I'll leave the call of including this up to you!

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