Doc necessity for superuser privileges to execute pg_import_system_collations()

Hi.

It seems that only superusers can execute pg_import_system_collations(), 
but this is not mentioned in the manual.

Since other functions that require superuser privileges describe that in 
the manual, I think it would be better to do the same for consistency.

Thoughts?

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Regards,

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Atsushi Torikoshi
NTT DATA CORPORATION