[Doc Patch] Clarify that CREATEROLE roles can GRANT default roles

Hi,

https://www.postgresql.org/docs/current/default-roles.html mentions the
"Administrator" several times, but does not specify it further. I
understand that it is mentioned elsewhere [1], but I think it would be
beneficial to remind the reader on that page at least once that
"Administrators" includes "roles with the CREATEROLE privilege" in the
context of GRANTing and REVOKEing default privileges, e.g. with the
attached patch.

Michael

[1] e.g. at https://www.postgresql.org/docs/current/sql-createrole.html
it is mentioned that CREATEROLE privs can be regarded as "almost-superuser-roles"

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Michael Banck
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