pg_restore documentation and --create/--single-transaction limitation

Hi All,
The pg_restore documentation usually mentions the pair of switches
which can not be used together. However, it does not mention that
--create and --single-transaction can not be used together. Here's a
patch fixing the same.

Looking for a precedence, I found that we have mentioned a similar
limitation concerning --data-only and --schema-only only under
--schema and not at both the sections. Maybe it's missing or we chose
to mention it only at one place. But then I am not sure which one
place I should use to mention the new limitation. So, I have added the
note in the sections corresponding to both the switches so that a user
reading either of them knows about the limitation.

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Best Wishes,
Ashutosh Bapat