Clarify advice for specific-over-generic impls

Author: jmaargh

library/core/src/ops/deref.rs

@@ -56,16 +56,19 @@
 /// types.
 ///
 /// Generic implementations, such as for [`Box<T>`][box] (which is generic over
-/// every type) should be careful to few methods, since the target type
-/// is unknown and therefore every method could collide with one on the target
-/// type, causing confusion for users. `Box<T>` has few methods (though
-/// several associated functions), partly for this reason.
-///
-/// Specific implementations, such as for [`String`][string] (which only
-/// implements for `Target = str`) can have many methods, since avoiding
+/// every type and dereferences to `T`) should be careful to provide few or no
+/// methods, since the target type is unknown and therefore every method could
+/// collide with one on the target type, causing confusion for users.
+/// `impl<T> Box<T>` has no methods (though several associated functions),
+/// partly for this reason.
+///
+/// Specific implementations, such as for [`String`][string] (whose `Deref`
+/// implementation has `Target = str`) can have many methods, since avoiding
 /// collision is much easier. `String` and `str` both have many methods, and
 /// `String` additionally behaves as if it has every method of `str` because of
-/// deref coercion.
+/// deref coercion. The implementing type may also be generic while the
+/// implementation is still specific in this sense; for example, [`Vec<T>`][vec]
+/// dereferences to `[T]`, so methods of `T` are not applicable.
 ///
 /// Consider also that deref coericion means that deref traits are a much larger
 /// part of a type's public API than any other trait as it is implicitly called
@@ -94,6 +97,7 @@
 /// [type coercions]: ../../reference/type-coercions.html
 /// [box]: ../../alloc/boxed/struct.Box.html
 /// [string]: ../../alloc/string/struct.String.html
+/// [vec]: ../../alloc/vec/struct.Vec.html
 /// [rc]: ../../alloc/rc/struct.Rc.html
 /// [cow]: ../../alloc/borrow/enum.Cow.html
 ///