Incorporate reviewer feedback

Author: xwu

userdocs/diagnostics/protocol-type-non-conformance.md

@@ -31,7 +31,7 @@ print(Cat.species)    // Prints "Felis catus"
 print(Animal.species) // Error
 ```
 
-Since a type conforms to a protocol only when it satisfies _all_ of that protocol's requirements, the existential type `Animal` does not conform to the protocol `Animal` because it cannot satisfy its own requirement for the static property `species`:
+Since a type conforms to a protocol only when it satisfies _all_ of that protocol's requirements, the existential type `Animal` does not conform to the protocol `Animal` because it cannot satisfy the protocol's requirement for the static property `species`:
 
 ```swift
 func declareAnimalSpecies<T: Animal>(_ animal: T) {
@@ -54,6 +54,8 @@ Currently, even if a protocol `P` requires no initializers or static members, th
 
 Concrete types that _do_ conform to protocols can provide functionality similar to that of existential types. For example, the standard library provides the `AnyHashable` type for `Hashable` values. Manual implementation of such __type erasure__ can require specific knowledge of the semantic requirements for each protocol involved and is beyond the scope of this discussion.
 
+For more on using existential types, see [Protocols as Types](https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID275) in _The Swift Programming Language_.
+
 ## Exceptions
 
-The Swift protocol `Error` has no requirements and, when used as a type, conforms to itself; `@objc` protocols with no static requirements can also be used as types that conform to themselves.
+The Swift protocol `Error` has no requirements and, when used as a type, conforms to itself; `@objc` protocols with no static requirements can also be used as types that conform to themselves.