PG15 beta1 fix pg_stats_ext/pg_stats_ext_exprs view manual

Hi hackers,
Thanks to all the developers. The attached patch updates the manual for the pg_stats_ext and pg_stats_ext_exprs view.
The current pg_stats_ext/pg_stats_ext_exprs view manual are missing the inherited column. This column was added at the
sametime as the stxdinherit column in the pg_statistic_ext_data view. The attached patch adds the missing description.
Ifthere is a better description, please correct it. 

Commit: Add stxdinherit flag to pg_statistic_ext_data
    https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=269b532aef55a579ae02a3e8e8df14101570dfd9
Current Manual:
    https://www.postgresql.org/docs/15/view-pg-stats-ext.html
    https://www.postgresql.org/docs/15/view-pg-stats-ext-exprs.html

Regards,
Noriyoshi Shinoda