slight tweaks to documentation about runtime pruning

Hi,

Documentation of run-time pruning tells readers to inspect "nloops"
property of the EXPLAIN ANALYZE output, but I think that's a typo of
"loops" which is actually output ("internal variable to track that
property is indeed nloops).

However, for pruned partitions' subplans, what's actually shown is the
string "(never executed)", not loops.  So, wouldn't it be better to tell
the readers to look for that instead of "loops"?

Attached is what I have in mind.

(cc'ing David Rowley to get his opinion, as he presumably wrote that line.)

Thanks,
Amit