Document support for Web Identity Token in DefaultCredentialsProvider (#2667)

Co-authored-by: Debora N. Ito <[email protected]>

Author: jamieliu386

core/auth/src/main/java/software/amazon/awssdk/auth/credentials/DefaultCredentialsProvider.java

@@ -26,6 +26,7 @@
  * <ol>
  *   <li>Java System Properties - <code>aws.accessKeyId</code> and <code>aws.secretAccessKey</code></li>
  *   <li>Environment Variables - <code>AWS_ACCESS_KEY_ID</code> and <code>AWS_SECRET_ACCESS_KEY</code></li>
+ *   <li>Web Identity Token credentials from system properties or environment variables</li>
  *   <li>Credential profiles file at the default location (~/.aws/credentials) shared by all AWS SDKs and the AWS CLI</li>
  *   <li>Credentials delivered through the Amazon EC2 container service if AWS_CONTAINER_CREDENTIALS_RELATIVE_URI" environment
  *   variable is set and security manager has permission to access the variable,</li>