Document bitmap transparency and public API

Explain image behavior for each credential tile version, package the README and XML documentation, and complete the public API comments.

src/Lithnet.CredentialProvider/Controls/InsecurePasswordTextboxControl.cs

@@ -12,13 +12,13 @@ namespace Lithnet.CredentialProvider
         private string password;
 
         /// <summary>
-        /// Creates a new <c ref="InsecurePasswordTextboxControl"/> control
+        /// Creates a new <see cref="InsecurePasswordTextboxControl"/> control.
         /// </summary>
         /// <param name="key">The unique key for this control</param>
         public InsecurePasswordTextboxControl(string key) : this(key, null) { }
 
         /// <summary>
-        /// Creates a new <c ref="InsecurePasswordTextboxControl"/> control
+        /// Creates a new <see cref="InsecurePasswordTextboxControl"/> control.
         /// </summary>
         /// <param name="key">The unique key for this control</param>
         /// <param name="label">The label associated with the control</param>
@@ -66,4 +66,4 @@ namespace Lithnet.CredentialProvider
             return clone;
         }
     }
-}
+}