[ruby/irb] Use markdown format for docs

(https://github.com/ruby/irb/pull/890)

* Convert irb.rb's document into markdown format

* Hide should-be-private top-level methods from docs

* Skip xmp.rb's docs

* Declare lib/irb.rb's markup do it works in ruby/ruby too

* Ignore docs folder

https://github.com/ruby/irb/commit/e9a175e06b

lib/irb/ext/multi-irb.rb

@@ -5,7 +5,7 @@
 #
 
 module IRB
-  class JobManager
+  class JobManager # :nodoc:
 
     # Creates a new JobManager object
     def initialize
@@ -166,12 +166,12 @@ module IRB
   @JobManager = JobManager.new
 
   # The current JobManager in the session
-  def IRB.JobManager
+  def IRB.JobManager # :nodoc:
     @JobManager
   end
 
   # The current Context in this session
-  def IRB.CurrentContext
+  def IRB.CurrentContext # :nodoc:
     IRB.JobManager.irb(Thread.current).context
   end
 
@@ -179,7 +179,7 @@ module IRB
   #
   # The optional +file+ argument is given to Context.new, along with the
   # workspace created with the remaining arguments, see WorkSpace.new
-  def IRB.irb(file = nil, *main)
+  def IRB.irb(file = nil, *main) # :nodoc:
     workspace = WorkSpace.new(*main)
     parent_thread = Thread.current
     Thread.start do

lib/irb/help.rb

@@ -6,7 +6,7 @@
 
 module IRB
   # Outputs the irb help message, see IRB@Command-Line+Options.
-  def IRB.print_usage
+  def IRB.print_usage # :nodoc:
     lc = IRB.conf[:LC_MESSAGES]
     path = lc.find("irb/help-message")
     space_line = false