Triggers: Clarify the difference with triggers in RDBMS in the documentation

[dumbbell]

This fear comes from some feedback on the Erlang mailing-list:

Triggers in an RDBMS database are a nightmare. I never use trigger in MySQL / PostgreSQL / etc RDBMS.
I worried the trigger feature in Khepri will also become a nightmare. Please reconsider adding a trigger feature.

After giving a clearer explanation of what triggers in Khepri are for, the person felt reassured:

Thank you so much for your awesome clear explanation.

Triggers in Khepri could be compared to a combination between
erlang:monitor/2 (which would know about tree nodes) and erlang:apply/2.

I feel more comfortable now knowing this comparison.

Therefore, I need to write a section in the documentation to clarify what triggers in Khepri are and how they are similar to and different from triggers in RDBMS.