doc: Add Javadocs for setters in IdTokenCredentials.Builder (#1909)

* chore(oauth2_http): Improve IdTokenCredentials quality and docs

* chore: Fix lint issues

Original-PR: googleapis/google-auth-library-java#1909

Author: lqiu96

google-auth-library-java/oauth2_http/java/com/google/auth/oauth2/IdTokenCredentials.java

@@ -105,9 +105,9 @@ public class IdTokenCredentials extends OAuth2Credentials {
 
   private static final long serialVersionUID = -2133257318957588431L;
 
-  private IdTokenProvider idTokenProvider;
+  private final IdTokenProvider idTokenProvider;
+  private final List<IdTokenProvider.Option> options;
   private String targetAudience;
-  private List<IdTokenProvider.Option> options;
 
   private IdTokenCredentials(Builder builder) {
     this.idTokenProvider = Preconditions.checkNotNull(builder.getIdTokenProvider());
@@ -131,7 +131,7 @@ public IdToken getIdToken() {
 
   @Override
   public int hashCode() {
-    return Objects.hash(options, targetAudience);
+    return Objects.hash(idTokenProvider, options, targetAudience);
   }
 
   @Override
@@ -146,7 +146,8 @@ public boolean equals(Object obj) {
     }
     IdTokenCredentials other = (IdTokenCredentials) obj;
     return Objects.equals(this.idTokenProvider, other.idTokenProvider)
-        && Objects.equals(this.targetAudience, other.targetAudience);
+        && Objects.equals(this.targetAudience, other.targetAudience)
+        && Objects.equals(this.options, other.options);
   }
 
   @Override
@@ -169,16 +170,29 @@ public static class Builder extends OAuth2Credentials.Builder {
 
     protected Builder() {}
 
+    /**
+     * Sets the provider for the ID token.
+     *
+     * @param idTokenProvider the provider for the ID token, cannot be null
+     * @return the builder object
+     */
     @CanIgnoreReturnValue
     public Builder setIdTokenProvider(IdTokenProvider idTokenProvider) {
-      this.idTokenProvider = idTokenProvider;
+      this.idTokenProvider = Preconditions.checkNotNull(idTokenProvider);
       return this;
     }
 
     public IdTokenProvider getIdTokenProvider() {
       return this.idTokenProvider;
     }
 
+    /**
+     * Sets the target audience for the ID token.
+     *
+     * @param targetAudience the target audience, cannot be null for non-UserCredentials. If set for
+     *     UserCredentials, the value will be ignored.
+     * @return the builder object
+     */
     @CanIgnoreReturnValue
     public Builder setTargetAudience(String targetAudience) {
       this.targetAudience = targetAudience;
@@ -189,6 +203,12 @@ public String getTargetAudience() {
       return this.targetAudience;
     }
 
+    /**
+     * Sets the options for the ID token.
+     *
+     * @param options list of options, can be null or empty if no options are needed.
+     * @return the builder object
+     */
     @CanIgnoreReturnValue
     public Builder setOptions(List<IdTokenProvider.Option> options) {
       this.options = options;