feat: Adding documentation comments.

Sources/ConfigurationClient/Constants.swift

@@ -1,3 +1,5 @@
+/// Represents keys in the environment that can be used to locate a user's
+/// configuration file.
 @_spi(Internal)
 public enum EnvironmentKey {
   static let xdgConfigHome = "XDG_CONFIG_HOME"
@@ -5,6 +7,7 @@ public enum EnvironmentKey {
   static let hpaConfigFile = "HPA_CONFIG_FILE"
 }
 
+/// Represents keys that are used internally for directory names, file names, etc.
 @_spi(Internal)
 public enum HPAKey {
   public static let configDirName = "hpa"