Documented the Pathable protocol in the library target.

Library/Sources/Internal/Protocols/Pathable.swift

@@ -1,7 +1,9 @@
+/// A type that can provide a relative path representation.
 protocol Pathable {
     
     // MARK: Properties
     
+    /// A read-only relative path representation.
     var path: String { get }
     
 }