[foreign-memaccess] RFR 8226527: Add package-info javadoc

[Maurizio Cimadamore]

Hi
i've done a comprehensive pass over the javadoc of the memory access 
API, to add a package-level javadoc, and also to improve the quality of 
the javadoc pretty much across the board.

The patch is here:

http://cr.openjdk.java.net/~mcimadamore/panama/8226527/

Or, if you prefer, a link to the new API javadoc is here:

http://cr.openjdk.java.net/~mcimadamore/panama/memaccess_javadoc_v2

I've also did some minor tweaks on some API points, to make them more 
consistent - to name a few:

- renamed GroupLayout.struct/union to ofStruct/ofUnion
- renamed the methods in MemoryAccessVarHandle to make it more 
consistent with naming convention in the combinators defined in 
MethodHandles
- removed references to 'endianness' and replaced with 'order' (this is 
what ByteBuffer does, I think it's better to stick with existing trends)
- removed some of the 'Kind' enums which were not adding a lot of value. 
They are still there in the impl, but they are no longer exposed. We can 
always do so e.g. if we decided to add way more value layout kinds.

The resulting API feels pleasingly compact and tight (but yet expressive).

Comments welcome.

Maurizio