Update documentation.md (#259)
Update documentation.md (#259)

* Update documentation.md

added "Note that you can use `is` attribute with any HTML tags, but not with [`template` tag](#fragments-loops)." as suggested in https://github.com/riot/riot/issues/2742#issuecomment-517576194

* Update documentation.md

add note about call dynamically function in handler by using this[props.myfunc] form.
https://github.com/riot/riot/issues/2743#issuecomment-516089635

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