docs: Update docs for GoogleCredentialsProvider#setScopesToApply (#4057)

[GoogleCredentialsProvider#setScopesToApply](https://github.com/googleapis/sdk-platform-java/blob/343a8ef9fe2db8897f83a865805cb1a04eb1be5a/gax-java/gax/src/main/java/com/google/api/gax/core/GoogleCredentialsProvider.java#L123)
would only work if the underlying credentials require the scopes to be
set. Update the docs to reflect the intended behavior.

Fixes googleapis/sdk-platform-java#3738

Author: blakeli0

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/core/GoogleCredentialsProvider.java

@@ -118,7 +118,9 @@ public abstract static class Builder {
 
     /**
      * Sets the scopes to apply to the credentials that are acquired from Application Default
-     * Credentials, before the credentials are sent to the service.
+     * Credentials, before the credentials are sent to the service. These scopes only take effect if
+     * the underlying credentials require them. See {@link GoogleCredentials#createScopedRequired()}
+     * for more details on when scopes are necessary.
      */
     public abstract Builder setScopesToApply(List<String> val);