[spec/struct] Tweak Copy Constructor docs

Change headings to be independent of Struct Constructor section, because
copy ctor section is quite long with subheadings that should be shown in
the TOC.
Move sentence up about POD.
Make examples runnable.
Add Disabled Copying subheading.
Minor tweaks.
Add example for union with field of type struct which has a copy ctor.

Author: ntrel

spec/struct.dd

@@ -1055,10 +1055,11 @@ $(H3 $(LNAME2 field-init, Field initialization inside a constructor))
         ---
         )
 
-$(H3 $(LEGACY_LNAME2 StructCopyConstructor, struct-copy-constructor, Struct Copy Constructors))
+$(H2 $(LEGACY_LNAME2 StructCopyConstructor, struct-copy-constructor, Struct Copy Constructors))
 
     $(P Copy constructors are used to initialize a `struct` instance from
-    another `struct` of the same type.)
+    another instance of the same type. A `struct` that defines a copy constructor
+    is not $(RELATIVE_LINK2 POD, POD).)
 
     $(P A constructor declaration is a copy constructor declaration if it meets
     the following requirements:)
@@ -1094,6 +1095,8 @@ $(H3 $(LEGACY_LNAME2 StructCopyConstructor, struct-copy-constructor, Struct Copy
 
     $(OL
     $(LI When a variable is explicitly initialized:)
+
+    $(SPEC_RUNNABLE_EXAMPLE_COMPILE
     ---
     struct A
     {
@@ -1106,8 +1109,11 @@ $(H3 $(LEGACY_LNAME2 StructCopyConstructor, struct-copy-constructor, Struct Copy
         A b = a; // copy constructor gets called
     }
     ---
+    )
 
     $(LI When a parameter is passed by value to a function:)
+
+    $(SPEC_RUNNABLE_EXAMPLE_COMPILE
     ---
     struct A
     {
@@ -1122,9 +1128,12 @@ $(H3 $(LEGACY_LNAME2 StructCopyConstructor, struct-copy-constructor, Struct Copy
         fun(a);    // copy constructor gets called
     }
     ---
+    )
 
-    $(LI When a parameter is returned by value from a function and Named Returned Value Optiomization (NRVO)
+    $(LI When a parameter is returned by value from a function and Named Returned Value Optimization (NRVO)
     cannot be performed:)
+
+    $(SPEC_RUNNABLE_EXAMPLE_COMPILE
     ---
     struct A
     {
@@ -1150,11 +1159,15 @@ $(H3 $(LEGACY_LNAME2 StructCopyConstructor, struct-copy-constructor, Struct Copy
     }
     ---
     )
+    )
+
+$(H3 $(LNAME2 disable-copy, Disabled Copying))
 
-    $(LNAME2 disable-copy)
     $(P When a copy constructor is defined for a `struct` (or marked `@disable`), the compiler no
     longer implicitly generates default copy/blitting constructors for that `struct`:
     )
+
+    $(SPEC_RUNNABLE_EXAMPLE_FAIL
     ---
     struct A
     {
@@ -1170,7 +1183,9 @@ $(H3 $(LEGACY_LNAME2 StructCopyConstructor, struct-copy-constructor, Struct Copy
         fun(a);        // error: copy constructor cannot be called with types (immutable) immutable
     }
     ---
+    )
 
+    $(SPEC_RUNNABLE_EXAMPLE_FAIL
     ---
     struct A
     {
@@ -1183,19 +1198,40 @@ $(H3 $(LEGACY_LNAME2 StructCopyConstructor, struct-copy-constructor, Struct Copy
         A b = a; // error: copy constructor is disabled
     }
     ---
+    )
 
-    $(P If a `union S` has fields that define a copy constructor, whenever an object of type `S`
+    $(P If a `union U` has fields that define a copy constructor, whenever an object of type `U`
     is initialized by copy, an error will be issued. The same rule applies to overlapped fields
     (anonymous unions).)
 
-    $(P A `struct` that defines a copy constructor is not $(RELATIVE_LINK2 POD, POD).)
+    $(SPEC_RUNNABLE_EXAMPLE_FAIL
+    ---
+    struct S
+    {
+        this(ref S);
+    }
 
-$(H4 $(LNAME2 copy-constructor-attributes, Copy Constructor Attributes))
+    union U
+    {
+        S s;
+    }
+
+    void main()
+    {
+        U a;
+        U b = a; // error, could not generate copy constructor for U
+    }
+    ---
+    )
+
+$(H3 $(LNAME2 copy-constructor-attributes, Copy Constructor Attributes))
 
     $(P The copy constructor can be overloaded with different qualifiers applied
     to the parameter (copying from a qualified source) or to the copy constructor
     itself (copying to a qualified destination):
     )
+
+    $(SPEC_RUNNABLE_EXAMPLE_COMPILE
     ---
     struct A
     {
@@ -1216,10 +1252,13 @@ $(H4 $(LNAME2 copy-constructor-attributes, Copy Constructor Attributes))
         immutable A a5 = ia;    // calls 4
     }
     ---
+    )
 
     $(P The `inout` qualifier may be applied to the copy constructor parameter in
     order to specify that mutable, `const`, or `immutable` types are treated the same:
     )
+
+    $(SPEC_RUNNABLE_EXAMPLE_COMPILE
     ---
     struct A
     {
@@ -1238,8 +1277,9 @@ $(H4 $(LNAME2 copy-constructor-attributes, Copy Constructor Attributes))
         immutable(A) c = r3;
     }
     ---
+    )
 
-$(H4 $(LNAME2 implicit-copy-constructors, Implicit Copy Constructors))
+$(H3 $(LNAME2 implicit-copy-constructors, Implicit Copy Constructors))
 
     $(P A copy constructor is generated implicitly by the compiler for a `struct S`
     if all of the following conditions are met:)
@@ -1251,6 +1291,7 @@ $(H4 $(LNAME2 implicit-copy-constructors, Implicit Copy Constructors))
     )
 
     $(P If the restrictions above are met, the following copy constructor is generated:)
+
     ---
     this(ref return scope inout(S) src) inout
     {
@@ -1270,7 +1311,7 @@ $(GNAME Postblit):
     $(D this $(LPAREN) this $(RPAREN)) $(GLINK2 function, MemberFunctionAttributes)$(OPT) $(GLINK2 function, FunctionBody)
 )
 
-    $(P WARNING: The postblit is considered legacy and is not recommended for new code.
+    $(P $(RED Warning): The postblit is considered legacy and is not recommended for new code.
     Code should use $(RELATIVE_LINK2 struct-copy-constructor, copy constructors)
     defined in the previous section. For backward compatibility reasons, a `struct` that
     explicitly defines both a copy constructor and a postblit will only use the postblit
@@ -1280,7 +1321,7 @@ $(GNAME Postblit):
     will have priority over the copy constructor.)
 
         $(P $(I Copy construction) is defined as initializing
-         a struct instance from another struct of the same type.
+         a struct instance from another instance of the same type.
          Copy construction is divided into two parts:)
 
         $(OL
@@ -1298,7 +1339,7 @@ $(GNAME Postblit):
         etc. For example:
         )
 
-        $(SPEC_RUNNABLE_EXAMPLE_RUN
+        $(SPEC_RUNNABLE_EXAMPLE_COMPILE
         ---
         struct S
         {