pgsql: Fix comments in heapam.c.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0d32511eca5aec205cb6b609638ea67129ef6665

Modified Files
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src/backend/access/heap/heapam.c | 18 ++++++++----------
1 file changed, 8 insertions(+), 10 deletions(-)