[spec] Add warning about struct .init not initializing context pointer (#3825)

Author: ntrel

spec/property.dd

@@ -115,6 +115,9 @@ static assert(Foo.init.a == 0);
 static assert(Foo.init.b == 7);
 ----------------
 )
+
+$(H3 $(LNAME2 init-vs-construction, `.init` vs Default Construction))
+
     $(P Note that $(D .init) produces a default initialized object, not a
     default constructed one. If there is a default constructor for an object,
     it may produce a different value.)
@@ -134,6 +137,8 @@ void main()
     }
     S s1;           // OK. S() correctly initialize its context pointer.
     S s2 = S();     // OK. same as s1
+    s1.foo();       // OK
+
     S s3 = S.init;  // Bad. the context pointer in s3 is null
     s3.foo();       // Access violation
 }

spec/struct.dd

@@ -2323,7 +2323,7 @@ $(H2 $(LNAME2 nested, Nested Structs))
         ---
         )
 
-    $(P The static attribute will prevent a struct from being nested. As such,
+    $(P The `static` attribute will prevent a struct from being nested. As such,
         the struct will not have access to its enclosing scope.)
 
         $(SPEC_RUNNABLE_EXAMPLE_FAIL
@@ -2343,6 +2343,10 @@ $(H2 $(LNAME2 nested, Nested Structs))
         ---
         )
 
+    $(P $(B Warning): For nested structs, $(DDSUBLINK spec/property, init-vs-construction,
+        `.init` is not the same as default construction).)
+
+
 $(H2 $(LNAME2 unions_and_special_memb_funct, Unions and Special Member Functions))
 
     $(P Unions may not have postblits, destructors, or invariants.)