[spec/function] Add Hidden Parameters section (#3375)

Link to `this` parameter section in expression.dd.
Mention D-style variadic hidden parameters.

Author: ntrel

spec/class.dd

@@ -225,7 +225,7 @@ $(H2 $(LNAME2 super_class, Super Class))
 $(H2 $(LNAME2 member-functions, Member Functions (a.k.a. Methods)))
 
         $(P Non-static member functions have an extra hidden parameter
-        called $(I this) through which the class object's other members
+        called $(DDSUBLINK spec/expression, this, `this`) through which the class object's other members
         can be accessed.
         )
 

spec/const3.dd

@@ -343,7 +343,8 @@ $(H2 $(LNAME2 immutable_member_functions, Immutable Member Functions))
 
         $(P
         Immutable member functions are guaranteed that the object
-        and anything referred to by the $(CODE this) reference is immutable.
+        and anything referred to by the $(DDSUBLINK spec/expression, this, `this` reference)
+        is immutable.
         They are declared as:
         )
 
@@ -407,7 +408,7 @@ $(H2 $(LNAME2 const_member_functions, Const Member Functions))
         $(P
         Const member functions are functions that are not allowed to
         change any part of the object through the member function's
-        this reference.
+        $(DDSUBLINK spec/expression, this, `this` reference).
         )
 
 

spec/function.dd

@@ -2382,18 +2382,22 @@ $(H4 $(LNAME2 lazy_variadic_functions, Lazy Variadic Functions))
         parameter. This will prevent a closure being generated for the delegate,
         as `scope` means the delegate will not escape the function.)
 
-$(H3 $(LNAME2 this-reference, `this` Reference))
+$(LEGACY_LNAME2 this-reference)
+$(H3 $(LNAME2 hidden-parameters, Hidden Parameters))
 
-        $(P
+        $(UL
+        $(LI
             Non-static member functions all have a hidden parameter called the
-            `this` reference, which refers to the object for which
+            $(DDSUBLINK spec/expression, this, `this` reference), which refers to the object for which
             the function is called.
         )
-
-        $(P
+        $(LI D-style variadic functions have
+            $(RELATIVE_LINK2 d_style_variadic_functions, hidden parameters).)
+        $(LI
             Functions with `Objective-C` linkage have an additional hidden,
             unnamed, parameter which is the selector it was called with.
         )
+        )
 
 
 $(H2 $(LEGACY_LNAME2 Local Variables, local-variables, Local Variables))