Clarify object safety rules for methods striked from the vtable

When trying to figure out how `async_trait` object safety worked and thinking about `Stream::next`'s object safety, I found it very hard to understand 

This clarifies that object-safety can still be possible even if a method is striked from the vtable, and that `self` and `where `Self: Sized` are the ONLY ways to do that.
This was only really explained clearly in the examples below, so this includes this clarification in the object safety rules in the beginning of the section


---
Separately, NonDispatchable seems like you are out of luck, but this pattern: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=5529e6424895cccda19d4dd81f389b69, where we impl the trait for &T where T: Trait + ?Sized is actually quite common in the std (albeit with `self` parameter's so you don't need the `(&obj_ref)` dance to get method resolution to be happy, see https://rust-lang.zulipchat.com/#narrow/stream/249502-wg-async-foundations.2Fstream-trait-rfc/topic/Object.20safety/near/226749056 for more info). 

This seems like a pattern we should explain somewhere, but is the reference the right place for it? I have not mentioned it in this PR, but would be curious what people think

Author: guswynn

src/items/traits.md

@@ -68,12 +68,14 @@ Object safe traits can be the base trait of a [trait object]. A trait is
 *object safe* if it has the following qualities (defined in [RFC 255]):
 
 * It must not require `Self: Sized`
-* All associated functions must either have a `where Self: Sized` bound, or
+* All associated functions must either have a `where Self: Sized` bound 
+ (reciever type of `self: Self`implies this), or
     * Not have any type parameters (although lifetime parameters are allowed),
       and
     * Be a [method] that does not use `Self` except in the type of the receiver.
+    * otherwise this method is non-dispatchable from a trait object
 * It must not have any associated constants.
-* All supertraits must also be object safe.
+* All [supertraits] must also be object safe.
 
 When there isn't a `Self: Sized` bound on a method, the type of a method
 receiver must be one of the following types:
@@ -322,9 +324,10 @@ fn main() {
 [_WhereClause_]: generics.md#where-clauses
 [bounds]: ../trait-bounds.md
 [trait object]: ../types/trait-object.md
-[RFC 255]: https://github.com/rust-lang/rfcs/blob/master/text/0255-object-safety.md
+[RFC 255]: https://github.com/rust-lang/rfcs/blob/master/text/0255-object-.md
 [associated items]: associated-items.md
 [method]: associated-items.md#methods
+[supertraits]: traits.md#supertraits
 [implementations]: implementations.md
 [generics]: generics.md
 [where clauses]: generics.md#where-clauses