(#16076) Ability to fill in pin explanation

Adds the ability to fill it in and sets a reasonable default.

manifests/pin.pp

@@ -3,6 +3,7 @@
 
 define apt::pin(
   $ensure     = present,
+  $explanation = "${::caller_module_name}: ${name}",
   $order      = '',
   $packages   = '*',
   $priority   = 0,

spec/defines/pin_spec.rb

@@ -50,7 +50,7 @@ describe 'apt::pin', :type => :define do
           'owner'   => 'root',
           'group'   => 'root',
           'mode'    => '0644',
-          'content' => "# #{title}\nPackage: #{param_hash[:packages]}\nPin: release a=#{param_hash[:release] || title}\nPin-Priority: #{param_hash[:priority]}\n",
+          'content' => "# #{title}\nExplanation: : #{title}\nPackage: #{param_hash[:packages]}\nPin: release a=#{param_hash[:release] || title}\nPin-Priority: #{param_hash[:priority]}\n",
         })
       }
     end

templates/pin.pref.erb

@@ -1,4 +1,5 @@
 # <%= name %>
+Explanation: <%= explanation %>
 Package: <%= packages %>
 Pin: <%= pin %>
 Pin-Priority: <%= priority %>