chore: cleanup javadoc warnings (#330)

* chore: cleanup javadoc warnings

* docs: update return docs

Author: chingor13

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

@@ -63,6 +63,7 @@ private IdToken(String tokenValue, JsonWebSignature jsonWebSignature) {
    * Creates an IdToken given the encoded Json Web Signature.
    *
    * @param tokenValue String representation of the ID token.
+   * @throws IOException if JWT token parsing fails
    * @return returns com.google.auth.oauth2.IdToken
    */
   public static IdToken create(String tokenValue) throws IOException {
@@ -74,6 +75,7 @@ public static IdToken create(String tokenValue) throws IOException {
    *
    * @param jsonFactory JsonFactory to use for parsing the provided token.
    * @param tokenValue String representation of the ID token.
+   * @throws IOException if JWT token parsing fails
    * @return returns com.google.auth.oauth2.IdToken
    */
   public static IdToken create(String tokenValue, JsonFactory jsonFactory) throws IOException {

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

@@ -80,6 +80,7 @@ public String getOption() {
    * @param options List of Credential specific options for for the token. For example, an IDToken
    *     for a ComputeEngineCredential can return platform specific claims if
    *     "ComputeEngineCredentials.ID_TOKEN_FORMAT_FULL" is provided as a list option.
+   * @throws IOException if token creation fails
    * @return IdToken object which includes the raw id_token, expiration and audience.
    */
   IdToken idTokenWithAudience(String targetAudience, List<Option> options) throws IOException;

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

@@ -123,6 +123,7 @@ public class ImpersonatedCredentials extends GoogleCredentials
    * @param lifetime Number of seconds the delegated credential should be valid for (up to 3600).
    * @param transportFactory HTTP transport factory, creates the transport used to get access
    *     tokens.
+   * @return new credentials
    */
   public static ImpersonatedCredentials create(
       GoogleCredentials sourceCredentials,
@@ -153,6 +154,7 @@ public static ImpersonatedCredentials create(
    *     targetPrincipal.
    * @param scopes Scopes to request during the authorization grant.
    * @param lifetime Number of seconds the delegated credential should be valid for (up to 3600).
+   * @return new credentials
    */
   public static ImpersonatedCredentials create(
       GoogleCredentials sourceCredentials,

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

@@ -89,7 +89,7 @@ public JwtClaims merge(JwtClaims other) {
    * ServiceAccountJwtAccessCredentials#jwtWithClaims(JwtClaims)} or {@link
    * JwtCredentials#jwtWithClaims(JwtClaims)}.
    *
-   * @return
+   * @return true if all required fields have been set; false otherwise
    */
   public boolean isComplete() {
     return getAudience() != null && getIssuer() != null && getSubject() != null;