Update javadoc boilerplate for experimental classes which are for public use

[jack-berg]

We have standard boiler plate for internal classes which are public which reads:

 * <p>This class is internal and is hence not for public use. Its APIs are unstable and can change
 * at any time.

But I wonder if we should adjust our boilerplate for these types of methods - internal because we make no compatibility guarantees, but acceptable for public consumption provided the user accepts the risks of breaking changes.

Maybe something like:

This class is internal and experimental. Its APIs are unstable and can change at any time. Its APIs (or a version of them) may be promoted to the public stable API in the future, but no guarantees are made.

Places this updated message would be relevant:

• SdkTracerProviderUtil
• SdkMeterProviderUtil
• MeterConfig, TracerConfig, LoggerConfig
• SdkEventLoggerProvider
• ... and more

Originally posted by @jack-berg in #6367 (comment)