array mapping doco

[Kevin Ryde]

In the "Array Mapping" section of the 1.6.1 manual, I think it'd be a
lot clearer if "ra" in the prototypes was the same as "ARRAY" in the
text.  It's quite obscure until you twig to that, especially with
"lra" and "ARRAY1 ..." (there not being anything to do with a "1" in
the prototype).

For array-map!, perhaps ARRAY0 could be written DST-ARRAY or similar
to avoid any confusion with the sources.  The text could also probably
have the sentence "PROC is applied ..." first, to get the primary
action stated nice and prominently.