RFR (14) 8236769: Clarify javadoc of memory access API

[Maurizio Cimadamore]

Hi,
this is a small patch which addresses the javadoc issues in the memory 
access API, which were pointed out in the CSR review [1].

Additionally, this patch also adds a test (contributed by Paul Sandoz, 
thanks!), which makes sure that passing values of the wrong type to 
memory access var handle result in an CCE at runtime.

Since this is a doc & test fix, I've marked the issue as noreg-doc, and 
I think this should be ok for 14.

Cheers
Maurizio

[1] - https://bugs.openjdk.java.net/browse/JDK-8234050