Scripts

I searched the manual for how Trigger(COMBAT_RESULTS_TRIGGER) works, but I couldn't find anything about this function.

You must have opened the wrong manual. Here are the ones you should be looking at:
HOMM5_A2_Script_Functions.pdf
HOMM5_A2_IDs_for_Scripts.pdf
A2 indicates that it's the second expansion, i.e., "Tribes of the East." It added a lot of functions.

What exactly should be in the ID where the function CombatResult is located?

This is the battle identifier; you can use it to request various battle statistics from the game – who participated, who died.