RFR 8240957: Clarify BadAttributeValueExpException readObject method

[Roger Riggs]

The change for 8232622: Technical debt in BadAttributeValueExpException
failed to document the behavior of the readObject method to ensure 
compatibility.
Please review the javadoc for the readObject method.

Webrev:
    http://cr.openjdk.java.net/~rriggs/webrev-readobject-doc-8240957/

Thanks, Roger

[1] https://bugs.openjdk.java.net/browse/JDK-8232622
[2] CSR: https://bugs.openjdk.java.net/browse/JDK-8240314