vector instructions documentation

[Aldy Hernandez]

i just noticed the manual's section entitled:

	"Using vector instructions through built-in functions"

it documents specifying types through:

     typedef int v4si __attribute__ ((mode(V4SI)));

i should add documentation there for the vector_size() atribute.

Should i remove the above mode() documentation (for this section) in
favor of the new vector_size() documentation, or should i augment the
current docs by providing info for vector_size()?

In my opinion, we should go for the new vector_size attribute which is
much cleaner.

Aldy