[PATCH] PASSING clause documentation fix

Hello!

I noticed an old TODO while reading through the code for
the Section 8.14 docs. I've attached a patch below which
attempts to resolve it, by adding some more clarification
about the PASSING clause, now that it has been implemented.

Specifically, I clarified that jsonpath named variables ($varname)
can be set either through the vars parameter or using the
PASSING clause. Any feedback is appreciated!

Thanks,
Adi Gollamudi