Review of the PL/Tcl part: The functionality looks OK. There are some
cosmetic issues. If those are addressed, I think this can be committed.
In the documentation, "Event Triggers" -> "Event triggers".
For the example in the documentation, please show the output, that is,
what the trigger outputs.
Remove the extra space before " tclsnitch".
Document the return value (it is ignored). Will we need the return
value in a future expansion? Should we leave room for that?
Change "command trigger" to "event trigger" in several places.
compile_pltcl_function() does not catch trigger function being called as
event trigger or vice versa. Not sure if that should be covered.
The first PG_TRY block in pltcl_event_trigger_handler() is unnecessary,
because there is nothing in there that can throw an exception.
I'd remove some comments from pltcl_event_trigger_handler(). They were
obviously copied from pltcl_trigger_handler(), but that function is much
longer, so more comments might have been appropriate there.