Add documentation for GDScript when statement

tutorials/scripting/gdscript/gdscript_basics.rst

@@ -185,6 +185,8 @@ in case you want to take a look under the hood.
 +------------+---------------------------------------------------------------------------------------------------------------------------------------------------+
 | as         | Cast the value to a given type if possible.                                                                                                       |
 +------------+---------------------------------------------------------------------------------------------------------------------------------------------------+
+| when       | Used by :ref:`pattern guard expressions<doc_gdscript_basics_pattern_guards>` in match_ statements.                                                |
++------------+---------------------------------------------------------------------------------------------------------------------------------------------------+
 | self       | Refers to current class instance.                                                                                                                 |
 +------------+---------------------------------------------------------------------------------------------------------------------------------------------------+
 | super      | Resolves the scope of the parent method. See `Inheritance`_.                                                                                      |
@@ -1787,8 +1789,10 @@ The following pattern types are available:
             "Sword", "Splash potion", "Fist":
                 print("Yep, you've taken damage")
 
-Pattern guards
-""""""""""""""
+.. _doc_gdscript_basics_pattern_guards:
+
+Pattern guard expressions
+"""""""""""""""""""""""""
 
 Only one branch can be executed per ``match``. Once a branch is chosen, the rest are not checked.
 If you want to use the same pattern for multiple branches or to prevent choosing a branch with too general pattern,