Refer to relaxed trait bounds for ?

For `?`, in this branch, we had listed "questionably sized" in the
syntax index.  I'm not a fan of this terminology.  As I wrote
elsewhere, in Rust PR 145924:

> I don't think we should use the word "maybe" to refer to "?X" bound
> relaxations.  Either there's a predicate stating that a type
> parameter implements some trait or there isn't.  While of course I
> get it -- the type argument provided may or may not implement the
> trait -- I just think "maybe" is speaking to the wrong thing here.
> It focuses on the type argument when what makes more sense to focus
> on is the type parameter.  And the type parameter is not in a
> "maybe" state.

In this regard, I feel the same about "questionably" as I do "maybe".

We don't use this terminology elsewhere in the Reference, so let's not
use it here.  Let's instead refer simply to "relaxed trait bounds", as
we do elsewhere in the syntax index.

Author: traviscross

src/syntax-index.md

@@ -112,7 +112,7 @@ This appendix provides an index of tokens and common forms with links to where t
 | `<-`   | LArrow      | The left arrow symbol has been unused since before Rust 1.0, but it is still treated as a single token. |
 | `#`    | Pound       | [attributes], [raw string literals], [raw byte string literals], [raw C string literals] |
 | `$`    | Dollar      | [macros] |
-| `?`    | Question    | [try propagation expressions][question], [questionably sized][sized], [macro Kleene matcher] |
+| `?`    | Question    | [try propagation expressions][question], [relaxed trait bounds], [macro Kleene matcher] |
 | `~`    | Tilde       | The tilde operator has been unused since before Rust 1.0, but its token may still be used. |
 
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