This doc doesn't explain the meaning of the RPM scriptlets like %triggerin, %systemd_post and there is no example either. Some documentation like [1] will be appreciated,

The %triggerin script is run when a given target package is installed or upgraded..
To define one of these scripts, you need to list the name of the target package; for example:
%triggerin -- tcsh
script commands...
This example sets up a trigger for the tcsh package. If the tcsh package is installed or upgraded, RPM will run the script. If your package is installed or upgraded and the tcsh package is presently installed, RPM will also run the script.

[1] https://docs.fedoraproject.org/en-US/Fedora_Draft_Documentation/0.1/html/RPM_Guide/ch10s02.html