Document @:optional

From forum https://groups.google.com/forum/#!topic/haxelang/MXe0BbALOAw
It might be a good idea to place a bit info of `@:optional` in [this](http://haxe.org/manual/types-nullability-optional-arguments.html) and [this](http://haxe.org/manual/types-anonymous-structure.html) or [this](http://haxe.org/manual/type-system-typedef.html) page.

Review the forum discussion and the linked manual pages on nullable optional arguments, anonymous structures, and typedefs to determine where @:optional belongs. Update the relevant page or pages with a brief explanation, and consider the work done when the manual documents this feature clearly in the appropriate location.

Written by the indexing model from the issue text.