pgsql: Document some structures in attribute_stats.c

Document some structures in attribute_stats.c

Like relation_stats.c, these structures are used to track the argument
number, names and types of pg_restore_attribute_stats() and
pg_clear_attribute_stats().

Extracted from a larger patch by the same author, reworded by me for
consistency with relation_stats.c.

Author: Corey Huinker <corey.huinker@gmail.com>
Discussion: /messages/by-id/CADkLM=dpz3KFnqP-dgJ-zvRvtjsa8UZv8wDAQdqho=qN3kX0Zg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d6c132d83bff76acd569175af7f53b7c1bf429f4

Modified Files
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src/backend/statistics/attribute_stats.c | 10 ++++++++++
1 file changed, 10 insertions(+)