document lock obtained for FKs during detach

I failed to document the lock acquired on tables that reference the
partitioned table that we're detaching a partition from.  This patch
(proposed for backpatch to 12, where that feature was added) fixes that
problem.

Noticed the omission a week ago while working out some details of
concurrent detach.

-- 
Álvaro Herrera                            39°49'30"S 73°17'W
"It takes less than 2 seconds to get to 78% complete; that's a good sign.
A few seconds later it's at 90%, but it seems to have stuck there.  Did
somebody make percentages logarithmic while I wasn't looking?"
                http://smylers.hates-software.com/2005/09/08/1995c749.html