Clarify ATBs in syntax index

On the branch, we had an entry in the expressions table for
`Type<ident=Type>`.  This was notated as "explicit associated type
bounds" and referenced a TODO item about default type parameters not
being documented.

Perhaps what this means to describe are associated type bindings and
associated type bounds.  Let's document those in the type expressions
table.  We'll remove the TODO note, as it's not relevant to this.

Author: traviscross

src/syntax-index.md

@@ -175,7 +175,6 @@ This appendix provides an index of tokens and common forms with links to where t
 | `Trait::method(…)`<br>`Type::method(…)`<br>`<Type as Trait>::method(…)` | [disambiguated method calls] |
 | `Type<…>`                 | [generic arguments] (e.g. `Vec<u8>`) <!-- TODO: fix after generic args reorg --> |
 | `method::<…>(…)`<br>`path::<…>` | [generic arguments], aka turbofish |
-| `Type<ident=Type>`        | [explicit associated type bounds] (e.g. `Iterator<Item=T>`) <!-- undocumented https://github.com/rust-lang/reference/issues/24 --> |
 | `()`                      | [unit] |
 | `(expr)`                  | [parenthesized expressions] |
 | `(expr,)`                 | [single-element tuple expression] |
@@ -241,6 +240,8 @@ This appendix provides an index of tokens and common forms with links to where t
 | `impl Trait`                          | [impl trait types], [anonymous type parameters] |
 | `dyn Trait`                           | [trait object types] |
 | `ident`<br>`ident::…`                 | [type paths] (can refer to structs, enums, unions, aliases, traits, generics, etc.) |
+| `Trait<ident = Type>`                 | [associated type bindings] (e.g. `Iterator<Item = T>`) |
+| `Trait<ident: …>`                     | [associated type bounds] (e.g. `Iterator<Item: Send>`) |
 | `&Type`<br>`&mut Type`                | [reference types] |
 | `*mut Type`<br>`*const Type`          | [raw pointer types] |
 | `fn(…) -> Type`                       | [function pointer types] |
@@ -279,6 +280,8 @@ This appendix provides an index of tokens and common forms with links to where t
 [assembly operands]: asm.operand-type.supported-operands.in
 [assignment]: expr.assign
 [associated items]: items.associated
+[associated type bindings]: paths.expr
+[associated type bounds]: paths.expr
 [async blocks]: expr.block.async
 [async closures]: expr.closure.async
 [async functions]: items.fn.async
@@ -314,7 +317,6 @@ This appendix provides an index of tokens and common forms with links to where t
 [destructuring assignment]: expr.placeholder
 [disambiguated method calls]: expr.call.desugar
 [enumerations]: items.enum
-[explicit associated type bounds]: paths.expr
 [explicit crate paths]: paths.qualifiers.global-root
 [extern crate]: items.extern-crate
 [extern function pointer types]: type.fn-pointer.qualifiers