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

doc/src/sgml/ref/prepare.sgml

@@ -52,7 +52,7 @@ PREPARE <replaceable class="parameter">name</replaceable> [ ( <replaceable class
    parameter data types can optionally be specified. When a
    parameter's data type is not specified or is declared as
    <literal>unknown</literal>, the type is inferred from the context
-   in which the parameter is used (if possible). When executing the
+   in which the parameter is first referenced (if possible). When executing the
    statement, specify the actual values for these parameters in the
    <command>EXECUTE</command> statement.  Refer to <xref
    linkend="sql-execute"/> for more
@@ -104,7 +104,7 @@ PREPARE <replaceable class="parameter">name</replaceable> [ ( <replaceable class
       The data type of a parameter to the prepared statement.  If the
       data type of a particular parameter is unspecified or is
       specified as <literal>unknown</literal>, it will be inferred
-      from the context in which the parameter is used. To refer to the
+      from the context in which the parameter is first referenced. To refer to the
       parameters in the prepared statement itself, use
       <literal>$1</literal>, <literal>$2</literal>, etc.
      </para>