Clarify generics in method definitions and impls. Fixes #2679.

src/ch10-01-syntax.md

@@ -210,13 +210,20 @@ Here, we’ve defined a method named `x` on `Point<T>` that returns a reference
 to the data in the field `x`.
 
 Note that we have to declare `T` just after `impl` so we can use it to specify
-that we’re implementing methods on the type `Point<T>`.  By declaring `T` as a
+that we’re implementing methods on the type `Point<T>`. By declaring `T` as a
 generic type after `impl`, Rust can identify that the type in the angle
-brackets in `Point` is a generic type rather than a concrete type.
-
-We could, for example, implement methods only on `Point<f32>` instances rather
-than on `Point<T>` instances with any generic type. In Listing 10-10 we use the
-concrete type `f32`, meaning we don’t declare any types after `impl`.
+brackets in `Point` is a generic type rather than a concrete type. Because this
+is declaring the generic again, we could have chosen a different name for the
+generic parameter than the generic parameter declared in the struct definition,
+but using the same name is conventional. Methods written within an `impl` that
+declares the generic type will be defined on any instance of the type, no
+matter what concrete type ends up substituting for the generic type.
+
+The other option we have is defining methods on the type with some constraint
+on the generic type. We could, for example, implement methods only on
+`Point<f32>` instances rather than on `Point<T>` instances with any generic
+type. In Listing 10-10 we use the concrete type `f32`, meaning we don’t declare
+any types after `impl`.
 
 <span class="filename">Filename: src/main.rs</span>