Update request_method predicate documentation

I don't know if this is the best way to approach this situation, or if there is a better way to clean up this part of the docs, but this is something I just ran into.

I was looking for the `request_method` predicate and was sent to http://docs.pylonsproject.org/projects/pyramid/en/latest/narr/viewconfig.html

I remembered that it should be possible to pass a tuple, and was trying to verify that using `GET` would also automatically include `HEAD`, well that documentation is available at: http://docs.pylonsproject.org/projects/pyramid/en/latest/api/config.html#pyramid.config.Configurator.add_view

It seems the documentation has drifted apart, is there a way to simply pull the documentation from the source code and place it into that `viewconfig` narrative document? Is there a reason why that is duplicated?

I didn't verify any of the other predicates for inaccuracies, but I have a feeling that there is a bunch of gardening to be done here.

Cheers!