Fix comments in heapam.c.
After commits
85f6b49c2c and
3ba59ccc89, we can allow parallel inserts
which was earlier not possible as parallel group members won't conflict
for relation extension and page lock. In those commits, we forgot to
update comments at few places.
Author: Amit Kapila
Reviewed-by: Robert Haas and Dilip Kumar
Backpatch-through: 13
Discussion: https://postgr.es/m/CAFiTN-tMrQh5FFMPx5aWJ+1gi1H6JxktEhq5mDwCHgnEO5oBkA@mail.gmail.com