RFR (15) 8248011: Improve javadoc of Foreign Memory Access API

[Maurizio Cimadamore]

Hi,
this is a patch fix address several javadoc issues in the foreign memory 
API, following some internal reviews (mostly from Brian, thanks!).

The changes are aimed at reordering sections in the doc so that more 
advanced concepts are exposed last, and also to fix various issues in 
code samples (especially around MemoryLayout).

Since I was there I also took the opportunity of dropping a now-unused 
method in MemoryAddressImpl (copy).

Webrev:
http://cr.openjdk.java.net/~mcimadamore/8248011/

Cheers
Maurizio