Doc: improve docs about pg_statistic_ext_data.

Doc: improve docs about pg_statistic_ext_data.

Commit aa087ec64 was a bit over-hasty about the doc changes needed
while splitting pg_statistic_ext_data off from pg_statistic_ext.
It duplicated one para and inserted another in what seems to me
to be the wrong section.  Fix that up, and in passing do some minor
copy-editing.

Per report from noborusai.

Discussion: https://postgr.es/m/CAAM3qnLXLUz4mOBkqa8jxigpKhKNxzSuvwpjvCRPvO5EqWjxSg@mail.gmail.com