Document that errors are not output by log_statement (was they were in

8.0), and add as suggestion to use log_min_error_statement for this
purpose.  I also fixed the code so the first EXECUTE has it's prepare,
rather than the last which is what was in the current code.  Also remove
"protocol" prefix for SQL EXECUTE output because it is not accurate.

Backpatch to 8.1.X.