Fix order of steps in DISCARD ALL documentation
The docs have always been slightly inaccurate, but got particularly so
in
a874fe7b4c89, which made DISCARD ALL occur before everything else;
reorder.
Author: Jan Chochol
Discussion: https://postgr.es/m/CAEASf_3TzBbnXm64HpnD5zCZEh8An9jN8ubMR=De-vOXHMHGeA@mail.gmail.com