Documented the AppArguments protocol and the Environment+Properties extension in the library target.

Library/Sources/Public/Extensions/Environment+Properties.swift

@@ -4,6 +4,7 @@ extension Environment {
     
     // MARK: Computed
     
+    /// The logging level set in the environment variables.
     public var logLevel: String? {
         self.get("LOG_LEVEL")
     }

Library/Sources/Public/Protocols/AppArguments.swift

@@ -1,11 +1,15 @@
 import Logging
 
+/// A type that defines the input arguments that the `Hummingbird`app receives.
 public protocol AppArguments {
     
     // MARK: Properties
     
+    /// A bind address for the app.
     var hostname: String { get }
+    /// A logging level to configure for the app.
     var logLevel: Logger.Level? { get }
+    /// A port for the app to use,
     var port: Int { get }
     
 }