[DOCS] Clarify that f:constant returns an enum case

Using

    <f:constant name: '\Vendor\Package\Enum::CASE'>

returns the enum case Enum::CASE, not the value of
given (backed) enum case.

This change clarifies this.

See also:
https://www.php.net/manual/en/function.constant.php#example-3571

src/ViewHelpers/ConstantViewHelper.php

@@ -35,8 +35,8 @@
  *
  *    {f:constant(name: '\Vendor\Package\Class::CONSTANT')}
  *
- * Get enum value
- * --------------
+ * Get enum case
+ * -------------
  *
  * ::
  *