doc: Fix "Prev" link, take 2.

doc: Fix "Prev" link, take 2.

Commit 6678b58d78 fixed a wrong "Prev" link by changing the link
generation code to use [position()=last()] instead of [last()] in
the predicate on the union of reverse axes.  Unfortunately, that
caused documentation builds to take much longer.  To fix, combine
the "preceding" and "ancestor" steps into one "preceding" step and
one "ancestor" step, and revert the predicate back to [last()].
The smaller union evades the libxml2 bug while avoiding the build
time regression.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Tested-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1132496.1781718007%40sss.pgh.pa.us
Backpatch-through: 14