put contexts that force CTFE before its restrictions

It's more important to know when CTFE even happens than what exactly you
can and can't do.

Also, since this is the spec, make it a definitive list instead of some
examples.

Author: aG0aep6G

spec/function.dd

@@ -2181,10 +2181,45 @@ void main()
 $(H2 $(LNAME2 interpretation, Compile Time Function Execution (CTFE)))
 
     $(P Functions which are both portable and free of global side-effects can be
-    executed in Compile Time Function Execution. CTFE is subject to the
-    following restrictions:
+    executed at compile time. In certain contexts, such compile time execution
+    is guaranteed. It is called Compile Time Function Execution (CTFE) then.
+    The contexts that trigger CTFE are:)
+
+    $(UL
+    $(LI initialization of a static variable or a manifest constant)
+    $(LI static initializers of struct/class members)
+    $(LI dimension of a static array)
+    $(LI argument for a template value parameter)
+    $(LI $(D_KEYWORD static if))
+    $(LI $(D_KEYWORD static foreach))
+    $(LI $(D_KEYWORD static assert))
+    $(LI $(D_KEYWORD mixin) statement)
     )
 
+$(SPEC_RUNNABLE_EXAMPLE_COMPILE
+---
+template eval( A... )
+{
+    const typeof(A[0]) eval = A[0];
+}
+
+int square(int i)
+{
+    return i * i;
+}
+
+void foo()
+{
+    static j = square(3);      // CTFE
+    writeln(j);
+    assert(square(4));        // run time
+    writeln(eval!(square(5))); // CTFE
+}
+---
+)
+
+    $(P CTFE is subject to the following restrictions:)
+
     $(OL
     $(LI The function source code must be available to the compiler. Functions
         which exist in the source code only as $(D_KEYWORD extern) declarations
@@ -2277,39 +2312,6 @@ static assert(countTen(12) == 12);  // invalid, modifies y.
         is used.
     )
 
-
-    $(P In order to be executed in CTFE, the function
-    must appear in a context where it must be so executed, for
-    example:)
-
-    $(UL
-    $(LI initialization of a static variable or a manifest constant)
-    $(LI dimension of a static array)
-    $(LI argument for a template value parameter)
-    )
-
-$(SPEC_RUNNABLE_EXAMPLE_COMPILE
----
-template eval( A... )
-{
-    const typeof(A[0]) eval = A[0];
-}
-
-int square(int i)
-{
-    return i * i;
-}
-
-void foo()
-{
-    static j = square(3);      // compile time
-    writeln(j);
-    writeln(square(4));        // run time
-    writeln(eval!(square(5))); // compile time
-}
----
-)
-
     $(P Executing functions via CTFE can take considerably
     longer than executing it at run time.
     If the function goes into an infinite loop, it will hang at