[Amforth] Address arithmetic

[Enoch <ix...@ho...>]

Hello Matthias & all,

Just a quick doc comment. 

Amforth introduces the e-addr and f-addr concepts to match the AVR arch.

I think that it is worth mentioning that f-addr is word based and there
is no subtype to it.

That is unlike the data space relationship (quoting forth-rc1 page 22):
a-addr ⇒ c-addr ⇒ addr ⇒ u;

Thanks, Enoch.