Add first cut at Performer docs

lib/good_job/performer.rb

@@ -1,18 +1,46 @@
 module GoodJob
+  #
+  # Performer queries the database for jobs and performs them on behalf of a
+  # {Scheduler}. It mainly functions as glue between a {Scheduler} and the jobs
+  # it should be executing.
+  #
   class Performer
+    # @!attribute [r] name
+    # @return [String]
+    #   a meaningful name to identify the performer in logs and for debugging.
+    #   This is usually set to the list of queues the performer will query,
+    #   e.g. +"-transactional_messages,batch_processing"+.
     attr_reader :name
 
+    # @param target [Object]
+    #   An object that can perform jobs. It must respond to +method_name+ by
+    #   finding and performing jobs and is usually a {Job} query,
+    #   e.g. +GoodJob::Job.where(queue_name: ['queue1', 'queue2'])+.
+    # @param method_name [Symbol]
+    #   The name of a method on +target+ that finds and performs jobs.
+    # @param name [String]
+    #   A name for the performer to be used in logs and for debugging.
+    # @param filter [#call]
+    #   Used to determine whether the performer should be used in GoodJob's
+    #   current state. GoodJob state is a +Hash+ that will be passed as the
+    #   first argument to +filter+ and includes info like the current queue.
     def initialize(target, method_name, name: nil, filter: nil)
       @target = target
       @method_name = method_name
       @name = name
       @filter = filter
     end
 
+    # Find and perform any eligible jobs.
     def next
       @target.public_send(@method_name)
     end
 
+    # Tests whether this performer should be used in GoodJob's current state by
+    # calling the +filter+ callable set in {#initialize}. Always returns +true+
+    # if there is no filter.
+    # @return [Boolean] whether the performer's {#next} method should be
+    #   called in the current state.
     def next?(state = {})
       return true unless @filter.respond_to?(:call)