diff --git a/mrbgems/mruby-errno/mrblib/errno.rb b/mrbgems/mruby-errno/mrblib/errno.rb
index 52899396b..b85f705ee 100644
--- a/mrbgems/mruby-errno/mrblib/errno.rb
+++ b/mrbgems/mruby-errno/mrblib/errno.rb
@@ -1,14 +1,49 @@
 module Errno
+  #
+  # call-seq:
+  #   Errno.const_defined?(name) -> true or false
+  #
+  # Returns true if the given name is defined as an errno constant,
+  # false otherwise. This method checks both system-defined errno
+  # constants and those defined by the superclass.
+  #
+  #   Errno.const_defined?(:ENOENT)  #=> true
+  #   Errno.const_defined?(:EINVAL)  #=> true
+  #   Errno.const_defined?(:UNKNOWN) #=> false
+  #
   def Errno.const_defined?(name)
     __errno_defined?(name) or super
   end
 
+  #
+  # call-seq:
+  #   Errno.const_missing(name) -> errno_class
+  #
+  # Called when an undefined constant is referenced. This method
+  # attempts to define the errno constant if it exists in the system,
+  # otherwise delegates to the superclass.
+  #
+  #   Errno::ENOENT  # triggers const_missing if not yet defined
+  #   #=> Errno::ENOENT
+  #
   def Errno.const_missing(name)
     __errno_define(name) or super
   end
 
-  # Module#constants is defined in mruby-metaprog
-  # So, it may be raised NoMethodError
+  #
+  # call-seq:
+  #   Errno.constants -> array
+  #
+  # Returns an array of all errno constant names available on the system.
+  # This includes both already defined constants and those that can be
+  # dynamically defined.
+  #
+  #   Errno.constants
+  #   #=> [:EPERM, :ENOENT, :ESRCH, :EINTR, :EIO, ...]
+  #
+  # Note: Module#constants is defined in mruby-metaprog, so this method
+  # may raise NoMethodError if that gem is not available.
+  #
   def Errno.constants
     __errno_list(super)
   end