Doc: fix examples for pg_input_error_info().
These were causing "contents ... exceed the available area"
warnings in PDF builds, and also didn't quite follow our markup
conventions for function examples. To fix the overwidth
problem, reduce the number of fields shown in one example,
and also insert &zwsp; to let the header line be broken in
a reasonable place.
Discussion: https://postgr.es/m/
20230324194701.dqkzcdtlcikseo22@awork3.anarazel.de