doc: minor update for description of "pg_roles" view

Hi

Here:

   https://www.postgresql.org/docs/12/view-pg-roles.html

we state:

   "This view explicitly exposes the OID column of the underlying table,
    since that is needed to do joins to other catalogs."

I think it's superfluous to mention this now OIDs are exposed by default;
attached patch (for REL_12_STABLE and HEAD) simply removes this sentence.


Regards

Ian Barwick


-- 
  Ian Barwick                   https://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services