pgsql: postgres_fdw: Update some comments in functions for statistics i

postgres_fdw: Update some comments in functions for statistics import.

* Fix a typo in a comment in match_attrmap(): s/attname/remote_attname/
* Add a comment about using a NullableDatum array in
import_fetched_statistics().
* Tweak the comments for each of the two functions.

Follow-up to commits 28972b6fc and 54cd6fc83.

Author: Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>
Reviewed-by: Chao Li <lic(at)highgo(dot)com>
Discussion: https://postgr.es/m/CAPmGK17d%3DUz-_teKwZdJN8T644QQyzEq%3DpUuf9gc%2Bc2Hp3x6%2Bw%40mail.gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/debfb3c8a8c93e7bcc3638523b7ffc0846cc2db8

Modified Files
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contrib/postgres_fdw/postgres_fdw.c | 20 +++++++++++++-------
1 file changed, 13 insertions(+), 7 deletions(-)