Clarify the ra != rb case in compareJsonbContainers().

Clarify the ra != rb case in compareJsonbContainers().

It's impossible to reach this case with either ra or rb being
WJB_DONE, because our earlier checks that the structure and
length of the inputs match should guarantee that we reach their
ends simultaneously.  However, the comment completely fails to
explain this, and the Asserts don't cover it either.  The comment
is pretty obscure anyway, so rewrite it, and extend the Asserts
to reject WJB_DONE.

This is only cosmetic, so no need for back-patch.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/0c623e8a204187b87b4736792398eaf1@postgrespro.ru