pgsql: Doc: explain how to test ADMIN privilege with pg_has_role().

Doc: explain how to test ADMIN privilege with pg_has_role().

This has always been possible, but the syntax is a bit obscure,
and our user-facing docs were not very helpful.  Spell it out
more clearly.

Per complaint from Dominique Devienne.  Back-patch to
all supported branches.

Discussion: https://postgr.es/m/CAFCRh-8JNEy+dV4SXFOrWca50u+d=--TO4cq=+ac1oBtfJy4AA@mail.gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8e16522163efce0e2d75b6e8c4413be22e637550

Modified Files
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doc/src/sgml/func.sgml | 4 ++++
1 file changed, 4 insertions(+)