When a composite type being used in a typed table is modified by way
of ALTER TYPE, a table rewrite occurs appearing to come from ALTER TYPE.
The existing event_trigger.c code was unable to cope with that
and raised a spurious error.  The fix is just to accept that command
tag for the event, and document this properly.

Noted while fooling with deparsing of DDL commands.  This appears to be
an oversight in commit 618c9430.

Thanks to Mark Wong for documentation wording help.