[docs] "Retroactive protocol refinement" in generics manifesto.

"Retroactive protocol refinement" is an accurate title for the section.

"Conditional conformances via protocol extensions" is not accurate
because the protocol refinements may not be conditional.

Author: dan-zheng

docs/GenericsManifesto.md

@@ -694,9 +694,9 @@ foo(X())
 
 Under what circumstances should it print "P"? If `foo()` is defined within the same module as the conformance of `X` to `P`? If the call is defined within the same module as the conformance of `X` to `P`? Never? Either of the first two answers requires significant complications in the dynamic casting infrastructure to take into account the module in which a particular dynamic cast occurred (the first option) or where an existential was formed (the second option), while the third answer breaks the link between the static and dynamic type systems--none of which is an acceptable result.
 
-### Conditional conformances via protocol extensions
+### Retroactive protocol refinement
 
-We often get requests to make a protocol conform to another protocol. This is, effectively, the expansion of the notion of "Conditional conformances" to protocol extensions. For example:
+We often get requests to make protocols retroactively refine other protocols. For example:
 
 ```Swift
 protocol P {
@@ -707,8 +707,8 @@ protocol Q {
   func bar()
 }
 
-extension Q : P { // every type that conforms to Q also conforms to P
-  func foo() {    // implement "foo" requirement in terms of "bar"
+extension Q : P { // Make every type that conforms to Q also conforms to P
+  func foo() {    // Implement `P.foo` requirement in terms of `Q.bar`
     bar()
   }
 }