chore: Add javadoc for public method

Author: lqiu96

oauth2_http/java/com/google/auth/oauth2/GdchCredentials.java

@@ -276,6 +276,9 @@ static String getIssuerSubjectValue(String projectId, String serviceIdentityName
     return String.format("system:serviceaccount:%s:%s", projectId, serviceIdentityName);
   }
 
+  /**
+   * @return the projectId set in the GDCH SA Key file or the user set projectId
+   */
   @Override
   public final String getProjectId() {
     return projectId;

oauth2_http/java/com/google/auth/oauth2/ServiceAccountCredentials.java

@@ -798,6 +798,9 @@ public final String getServiceAccountUser() {
     return serviceAccountUser;
   }
 
+  /**
+   * @return the projectId set in the SA Key file or the user set projectId
+   */
   @Override
   public final String getProjectId() {
     return projectId;