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: https://postgr.es/m/CADkLM=dpz3KFnqP-dgJ-zvRvtjsa8UZv8wDAQdqho=qN3kX0Zg@mail.gmail.com