Reduce comments

Reduce the vertical space used by comments in ri_triggers.c, making
the file longer and more tedious to read than it needs to be.  Update
some comments to use a more common style.
Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/0ccdd3e1-10b0-dd05-d8a7-183507c11eb1%402ndquadrant.com