Update documentation on how to stub a puppet function

[TraGicCode]

I've noticed a reoccuring question of people asking how to stub puppet functions when using rspec-puppet. We should get some documentation on this going. Below are the 2 mechanisms i've used for stubbing puppet functions.

Approach 1

describe 'role::blah' do
  let(:pre_condition) do
    'function googlecloudstorage::generate_signed_url($x, $y, $z) { return \'https://asdasdasd.com/file.iso\' }'
  end
end

Approach 2

describe 'role::blah' do
  before(:each) do
          Puppet::Parser::Functions.newfunction(:pe_build_version, :type => :rvalue) {|args| '4.0.0'}
  end
end

Also based on conversations it sounds like the first approach is the one that most people should lean towards using for newer code.

[TraGicCode]

There could be a tile on “Stubbing Dependencies” or “Stubbing Puppet Functions” on the website docs http://rspec-puppet.com/documentation/. If not a new tile not sure which one to put it in….