Documentation for methods that do return structs by reference

[HertzDevil]

Structs are usually, but not always, passed by value, despite what this section says. Some methods that can return structs by references are:

• ReadInstanceVar (not actual methods per se, but the two are syntactically similar)
• Pointer#value
• defs whose body consists of a single InstanceVar
• defs whose body consists of a single self, unless the def has been re-instantiated (likely a bug)

Such a semantic difference could make or break a program, so we should document this conspicuously.