Update `doc/limitations.md` for argument destructuring.

doc/limitations.md

@@ -219,7 +219,7 @@ $ ruby -e 'def m(*r,**k) p [r,k] end; m("a"=>1,:b=>2)'
 [[{"a"=>1}], {:b=>2}]
 ```
 
-#### mruby []
+#### mruby [mruby 2.0.0]
 
 ```
 $ ./bin/mruby -e 'def m(*r,**k) p [r,k] end; m("a"=>1,:b=>2)'
@@ -227,3 +227,23 @@ trace (most recent call last):
 	[0] -e:1
 -e:1: keyword argument hash with non symbol keys (ArgumentError)
 ```
+
+## Argument Destructuring
+
+```ruby
+def m(a,(b,c),d); p [a,b,c,d]; end
+m(1,[2,3],4)  # => [1,2,3,4]
+```
+Destructured arguments (`b` and `c` in above example) cannot be accessed
+from the default expression of optional arguments and keyword arguments,
+since actual assignment is done after the evaluation of those default
+expressions. Thus:
+    
+```ruby
+def f(a,(b,c),d=b)
+  p [a,b,c,d]
+end
+f(1,[2,3])
+```
+
+CRuby gives `[1,2,3,nil]`. mruby raises `NoMethodError` for `b`.