Re: Completing PL support for Event Triggers

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.