Document the different forms of catalogs to help test writers

Without this patch it was not clear to me that the static compiler
operates primarily on a resource catalog.

This was a problem because I spent considerable amounts of time trying
to figure out the difference between resource[:source] returning
"puppet:///..." inside of
Puppet::Resource::Catalog::StaticCompiler#find when running `puppet
apply` and resource[:source] returning ["puppet:///..."] (an array)
when inside of #find from the spec test.

When testing catalog terminii the resource form of the catalog should be
created instead of the RAL form.

This patch addresses the problem by providing a description of the
difference between a resource catalog and a RAL catalog and when it is
appropriate to use each.