doc: Correct jsonpath string literal escapes description

The paragraph describing the JavaScript string literals allowed in
jsonpath expressions unnecessarily mentions JSON by erroneously
listing \v as allowed by JSON and mentioning the \xNN and \u{N...}
backslash escapes as deviations from JSON when in fact both are
accepted by ECMAScript/JavaScript.  Fix this by only referring to
JavaScript.

Author: Erik Wienhold <ewie@ewie.name>
Discussion: https://www.postgresql.org/message-id/flat/1EB17DF9-2636-484B-9DD0-3CAB19C4F5C4@justatheory.com

doc/src/sgml/json.sgml

@@ -791,21 +791,20 @@ UPDATE table_name SET jsonb_field[1]['a'] = '1';
    In particular, the way to write a double quote within an embedded string
    literal is <literal>\"</literal>, and to write a backslash itself, you
    must write <literal>\\</literal>.  Other special backslash sequences
-   include those recognized in JSON strings:
+   include those recognized in JavaScript strings:
    <literal>\b</literal>,
    <literal>\f</literal>,
    <literal>\n</literal>,
    <literal>\r</literal>,
    <literal>\t</literal>,
    <literal>\v</literal>
-   for various ASCII control characters, and
-   <literal>\u<replaceable>NNNN</replaceable></literal> for a Unicode
-   character identified by its 4-hex-digit code point.  The backslash
-   syntax also includes two cases not allowed by JSON:
+   for various ASCII control characters,
    <literal>\x<replaceable>NN</replaceable></literal> for a character code
-   written with only two hex digits, and
-   <literal>\u{<replaceable>N...</replaceable>}</literal> for a character
-   code written with 1 to 6 hex digits.
+   written with only two hex digits,
+   <literal>\u<replaceable>NNNN</replaceable></literal> for a Unicode
+   character identified by its 4-hex-digit code point, and
+   <literal>\u{<replaceable>N...</replaceable>}</literal> for a Unicode
+   character code point written with 1 to 6 hex digits.
   </para>
 
   <para>