let TIP and friends handle a multiline paragraph

Normally I would have done this in master, but there was already
a guide with wrapped content, so worked here. I am going to
cross-merge now.

railties/guides/rails_guides/generator.rb

@@ -204,7 +204,7 @@ module RailsGuides
         t = RedCloth.new(body)
         t.hard_breaks = false
         t.lite_mode = lite_mode
-        t.to_html(:notestuff, :plusplus, :code, :tip)
+        t.to_html(:notestuff, :plusplus, :code)
       end
     end
 

railties/guides/rails_guides/textile_extensions.rb

@@ -3,23 +3,24 @@ require 'active_support/core_ext/object/inclusion'
 module RailsGuides
   module TextileExtensions
     def notestuff(body)
-      body.gsub!(/^(IMPORTANT|CAUTION|WARNING|NOTE|INFO)[.:](.*)$/) do |m|
-        css_class = $1.downcase
-        css_class = 'warning' if css_class.in?(['caution', 'important'])
-
-        result = "<div class='#{css_class}'><p>"
-        result << $2.strip
-        result << '</p></div>'
-        result
-      end
-    end
-
-    def tip(body)
-      body.gsub!(/^TIP[.:](.*)$/) do |m|
-        result = "<div class='info'><p>"
-        result << $1.strip
-        result << '</p></div>'
-        result
+      # The following regexp detects special labels followed by a
+      # paragraph, perhaps at the end of the document.
+      #
+      # It is important that we do not eat more than one newline
+      # because formatting may be wrong otherwise. For example,
+      # if a bulleted list follows the first item is not rendered
+      # as a list item, but as a paragraph starting with a plain
+      # asterisk.
+      body.gsub!(/^(TIP|IMPORTANT|CAUTION|WARNING|NOTE|INFO)[.:](.*?)(\n(?=\n)|\Z)/m) do |m|
+        css_class = case $1
+                    when 'CAUTION', 'IMPORTANT'
+                      'warning'
+                    when 'TIP'
+                      'info'
+                    else
+                      $1.downcase
+                    end
+        %Q(<div class="#{css_class}"><p>#{$2.strip}</p></div>)
       end
     end