Add comment explaining fire_triggers=false in ri_PerformCheck()

Add comment explaining fire_triggers=false in ri_PerformCheck()

The reason for passing fire_triggers=false to SPI_execute_snapshot()
in ri_PerformCheck() was not documented, making it unclear why it was
done that way.  Add a comment explaining that it ensures AFTER triggers
on rows modified by the RI action are queued in the outer query's
after-trigger context and fire only after all RI updates on the same
row are complete.

Author: Yugo Nagata <nagata@sraoss.co.jp>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Surya Poondla <suryapoondla4@gmail.com>
Discussion: https://postgr.es/m/20250331212648.ad4ab804559001d7f0788741@sraoss.co.jp