Update the description of the documentation

Author: dingo-d

src/Standards/PSR12/Docs/Functions/ReturnTypeDeclarationStandard.xml

@@ -1,7 +1,9 @@
 <documentation title="Return Type Declaration">
     <standard>
     <![CDATA[
-    For functions and closures, the return type declaration must be correctly defined.
+    For function and closure return type declarations, there must be one space after the colon followed by the type declaration, and no space before the colon.
+
+    The colon and the return type declaration have to be on the same line as the argument list closing parenthesis.
     ]]>
     </standard>
     <code_comparison>