Expanded entries for generics.

* Now mentions method generics.
* Has separate entries for generic `fn`, `struct`, `enum`, and `impl`
  items.

(Thanks killercup).

Author: DanielKeep

src/doc/trpl/syntax-index.md

@@ -116,9 +116,12 @@
 
 <!-- Generics -->
 
-* `path<…>`: specifies parameters to generic type *in a type*.  See [Generics].
-* `path::<…>`: specifies parameters to generic type or function *in an expression*.
-* `ident<…>`: generic parameters.  See [Generics].
+* `path<…>` (*e.g.* `Vec<u8>`): specifies parameters to generic type *in a type*.  See [Generics].
+* `path::<…>`, `method::<…>` (*e.g.* `"42".parse::<i32>()`): specifies parameters to generic type, function, or method *in an expression*.
+* `fn ident<…> …`: define generic function.  See [Generics].
+* `struct ident<…> …`: define generic structure.  See [Generics].
+* `enum ident<…> …`: define generic enumeration.  See [Generics].
+* `impl<…> …`: define generic implementation.
 * `for<…> type`: higher-ranked lifetime bounds.
 * `type<ident=type>` (*e.g.* `Iterator<Item=T>`): a generic type where one or more associated types have specific assignments.  See [Associated Types].