Fix documentation for wal_summary_keep_time.
The documentation for this parameter lists its type as boolean, but
it is actually an integer. Furthermore, there is no mention of how
the value is interpreted when specified without units. This commit
fixes these oversights in commit
174c480508.
Co-authored-by: Hubert Depesz Lubaczewski
Discussion: https://postgr.es/m/ZZwkujFihO2uqKwp%40depesz.com