docs: Only first instance of a PREPARE parameter sets data type

docs:  Only first instance of a PREPARE parameter sets data type

If the first reference to $1 is "($1 = col) or ($1 is null)", the data
type can be determined, but not for "($1 is null) or ($1 = col)".  This
change documents this.

Reported-by: Morgan Owens

Discussion: https://postgr.es/m/153233728858.1404.15268121695358514937@wrigleys.postgresql.org

Backpatch-through: 9.3