Polish the public docs for MJIT [ci skip]

Now every private interface is cleaned up, and the public interface is
documented.

.document

@@ -18,6 +18,7 @@ gc.rb
 io.rb
 kernel.rb
 marshal.rb
+mjit.rb
 numeric.rb
 nilclass.rb
 pack.rb

mjit.rb

@@ -1,12 +1,15 @@
 module RubyVM::MJIT
+  # Return true if MJIT is enabled.
   def self.enabled?
     Primitive.cexpr! 'RBOOL(mjit_enabled)'
   end
 
+  # Stop generating JITed code.
   def self.pause(wait: true)
     Primitive.cexpr! 'mjit_pause(RTEST(wait))'
   end
 
+  # Start generating JITed code again after pause.
   def self.resume
     Primitive.cexpr! 'mjit_resume()'
   end
@@ -20,7 +23,9 @@ if RubyVM::MJIT.enabled?
     return # miniruby doesn't support MJIT
   end
 
-  RubyVM::MJIT::C = Object.new # forward declaration for ruby_vm/mjit/compiler
+  # forward declaration for ruby_vm/mjit/compiler
+  RubyVM::MJIT::C = Object.new # :nodoc:
+
   require 'ruby_vm/mjit/c_type'
   require 'ruby_vm/mjit/instruction'
   require 'ruby_vm/mjit/compiler'

tool/ruby_vm/views/lib/ruby_vm/mjit/instruction.rb.erb

@@ -1,4 +1,4 @@
-module RubyVM::MJIT
+module RubyVM::MJIT # :nodoc: all
   Instruction = Struct.new(
     :name,
     :bin,