[guides] guides:generate:kindle is the new interface to generate .mobi file instead of using KINDLE=1

guides/Rakefile

@@ -11,7 +11,7 @@ namespace :guides do
       ruby "rails_guides.rb"
     end
 
-    desc "Generate .mobi file"
+    desc "Generate .mobi file. The kindlegen executable must be in your PATH. You can get it for free from http://www.amazon.com/kindlepublishing"
     task :kindle do
       ENV['KINDLE'] = '1'
       Rake::Task['guides:generate:html'].invoke
@@ -59,14 +59,10 @@ Some arguments may be passed via environment variables:
   EDGE=1
     Indicate generated guides should be marked as edge.
 
-  KINDLE=1
-    Generate .mobi with all the guides.
-
 Examples:
   $ rake guides:generate ALL=1
-  $ rake guides:generate KINDLE=1
-  $ rake guides:generate:kindle
   $ rake guides:generate EDGE=1
+  $ rake guides:generate:kindle EDGE=1
   $ rake guides:generate GUIDES_LANGUAGE=es
     help
   end

guides/rails_guides/generator.rb

@@ -47,11 +47,6 @@
 #     Set to "1" to indicate generated guides should be marked as edge. This
 #     inserts a badge and changes the preamble of the home page.
 #
-#   KINDLE
-#     Set to "1" to generate the .mobi with all the guides. The kindlegen
-#     executable must be in your PATH. You can get it for free from
-#     http://www.amazon.com/kindlepublishing
-#
 # ---------------------------------------------------------------------------
 
 require 'set'

guides/source/ruby_on_rails_guides_guidelines.textile

@@ -97,13 +97,7 @@ h3. Kindle Guides
 
 h4(#generation-kindle). Generation
 
-To generate guides for the Kindle, you need to provide +KINDLE=1+ as an environment variable:
-
-<plain>
-KINDLE=1 bundle exec rake guides:generate
-</plain>
-
-or you can use the following task:
+To generate guides for the Kindle, use the following rake task:
 
 <plain>
 bundle exec rake guides:generate:kindle