docs: Only first instance of a PREPARE parameter sets data type
If the first reference to 1ドル is "(1ドル = col) or (1ドル is null)", the data
type can be determined, but not for "(1ドル is null) or (1ドル = col)". This
change documents this.
Reported-by: Morgan Owens
Discussion: https://postgr.es/m/
153233728858.1404.
15268121695358514937@wrigleys.postgresql.org
Backpatch-through: 9.3