RFR: JDK-8245958: j.l.Record need to mention that canonical constructor may not be public

[Vicente Romero]

Please review this API update. The API of java.lang.Record needs to be 
updated after the changes introduced by JEP 384: Records (Second 
Preview) [1], in particular this JEP changed the constraint that the 
canonical constructor had to be public. The associated bug is [2], the 
changeset is at [3] and the related CSR is at [4],

Thanks,
Vicente

[1] https://openjdk.java.net/jeps/384
[2] https://bugs.openjdk.java.net/browse/JDK-8245958
[3] http://cr.openjdk.java.net/~vromero/8245958/webrev.00/
[4] https://bugs.openjdk.java.net/browse/JDK-8246629