reflect/protoreflect: cleanup wording on Message.NewField

Change-Id: Id2200e335fb5caf8e7e1574524081fed36aab268
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/227817
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/reflect/protoreflect/value.go b/reflect/protoreflect/value.go
index a352ed9..f319810 100644
--- a/reflect/protoreflect/value.go
+++ b/reflect/protoreflect/value.go
@@ -114,8 +114,8 @@
 	// Mutable is a mutating operation and unsafe for concurrent use.
 	Mutable(FieldDescriptor) Value
 
-	// NewField returns a new value for assignable to the field of a given descriptor.
-	// For scalars, this returns the default value.
+	// NewField returns a new value that is assignable to the field
+	// for the given descriptor. For scalars, this returns the default value.
 	// For lists, maps, and messages, this returns a new, empty, mutable value.
 	NewField(FieldDescriptor) Value