Mention constant expression limitation prior to PHP 8.2 (#4413)

Co-authored-by: Gina Peter Banyard <[email protected]>

Author: exakat

language/enumerations.xml

@@ -192,8 +192,10 @@ enum Suit: string
   </para>
 
   <para>
-   Equivalent values must be literals or literal expressions. Constants and constant expressions
-   are not supported.  That is, <literal>1 + 1</literal> is allowed, but <literal>1 + SOME_CONST</literal>
+   The equivalent values may be a constant scalar expression.
+   Prior to PHP 8.2.0, the equivalent values had to be literals or literal expressions.
+   This means that constants and constant expressions were not supported.
+   That is, <code>1 + 1</code> was allowed, but not <code>1 + SOME_CONST</code>.
    is not.
   </para>