Determine exact log alerts for specific events

i do not think there is document listing all possible log messages. anyway, you may find some log messages in alertd mapping file i.e. /var/run/bigip_error_maps.dat. then, you may customize syslog-ng configuration to send Splunk specific log message.

sol6420: The /var/run/bigip_error_maps.dat file maps the alertd process input from the syslog-ng utility to an alert name

http://support.f5.com/kb/en-us/solutions/public/6000/400/sol6420.html

LTM 9.4.2+: Custom Syslog Configuration by Deb Allen

https://devcentral.f5.com/s/articles/LTM-9-4-2-Custom-Syslog-Configuration