Re: doc issues in event-trigger-matrix.html

I made a patch for this.  I have expanded the narrative discussion on 
what commands are supported for event triggers, also made a few 
corrections/additions there, based on inspecting the source code.  And 
then removed the big matrix, which doesn't provide any additional 
information, I think.

I think this is sufficient and covers everything.  The only hand-wavy 
thing I can see is exactly which ALTER commands trigger the sql_drop 
event.  But this was already quite imprecise before, and I think also 
not quite correct.  This might need a separate investigation.

In any case, we can use this as a starting point to iterate on the right 
wording etc.