doc: Some copy-editing around prefix operators

When postfix operators where dropped in 1ed6b8956, the CREATE OPERATOR
docs were not updated to make the RIGHTARG argument mandatory in the
grammar.

While at it, make the RIGHTARG docs more concise. Also, the operator
docs were mentioning "infix" in the introduction, while using "binary"
everywhere else.

Author: Christoph Berg <myon@debian.org>
Discussion: https://www.postgresql.org/message-id/flat/aAtpbnQphv4LWAye@msg.df7cb.de

doc/src/sgml/ref/create_operator.sgml

@@ -23,7 +23,7 @@ PostgreSQL documentation
 <synopsis>
 CREATE OPERATOR <replaceable>name</replaceable> (
     {FUNCTION|PROCEDURE} = <replaceable class="parameter">function_name</replaceable>
-    [, LEFTARG = <replaceable class="parameter">left_type</replaceable> ] [, RIGHTARG = <replaceable class="parameter">right_type</replaceable> ]
+    [, LEFTARG = <replaceable class="parameter">left_type</replaceable> ] , RIGHTARG = <replaceable class="parameter">right_type</replaceable>
     [, COMMUTATOR = <replaceable class="parameter">com_op</replaceable> ] [, NEGATOR = <replaceable class="parameter">neg_op</replaceable> ]
     [, RESTRICT = <replaceable class="parameter">res_proc</replaceable> ] [, JOIN = <replaceable class="parameter">join_proc</replaceable> ]
     [, HASHES ] [, MERGES ]
@@ -88,8 +88,8 @@ CREATE OPERATOR <replaceable>name</replaceable> (
 
   <para>
    For binary operators, both <literal>LEFTARG</literal> and
-   <literal>RIGHTARG</literal> must be defined.  For prefix operators only
-   <literal>RIGHTARG</literal> should be defined.
+   <literal>RIGHTARG</literal> must be defined.  For prefix operators, only
+   <literal>RIGHTARG</literal> must be defined.
    The <replaceable class="parameter">function_name</replaceable>
    function must have been previously defined using <command>CREATE
    FUNCTION</command> and must be defined to accept the correct number

doc/src/sgml/xoper.sgml

@@ -21,7 +21,7 @@
 
   <para>
    <productname>PostgreSQL</productname> supports prefix
-   and infix operators.  Operators can be
+   and binary (or infix) operators.  Operators can be
    overloaded;<indexterm><primary>overloading</primary><secondary>operators</secondary></indexterm>
    that is, the same operator name can be used for different operators
    that have different numbers and types of operands.  When a query is