Improve JavaDoc Session.getInstance(Properties, Authenticator)

**Describe the bug**
The class loading of providers changed if an Authenticator parameter is set for the call of `Session.getInstance(Properties, Authenticator)`. This is not documented and completely unexpected. It has cost me days to understand why with OAuth and Authenticator in production the provider was not loaded.

```
Class cl;
if (authenticator != null) {
cl = authenticator.getClass();
} else {
// Use implementation class, because that class loader has access to jakarta.mail module and implementation resources
cl = streamProvider.getClass();
}
```
**Expected behavior**
The same class loading behavior with and without Authenticator. The minimum is to documented this behavior.
For consistence with other parts of the API the context class loader should be used.