[DOCS] add details in trigger's infinite recursion warnings

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/10/static/trigger-definition.html
Description:

https://www.postgresql.org/docs/10/static/trigger-definition.html reads "It
is the trigger programmer's responsibility to avoid infinite recursion in
such scenarios." It would be neat to mention here pg_trigger_depth() which
is somewhat barely known.

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