Added a note on usage of textarea element

[aleator]

Maybe everyone else just knew about how to properly use textarea elements, but I spent half an hour trying the wrong way and got bit frustrated with them.

Setting the text area contents through child elements fails when the textarea elements are removed. Regardless of which element is removed, the content of the last text area vanishes and the remaining text areas get the previous contents in the original order. That is, the textarea contents jump from one element to another.

[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[OvermindDL1]

Is there ever a good case to put child nodes in a textarea, or should that argument be entirely removed? I've never seen it done nor done it myself, and removing the List of Html children would prevent this error in the future. If someone really needed to attach children they could just call node "textarea" anyway as an advanced case.

[aleator]

I would recommend changing the textarea to omit the child nodes.
The spec says that "The textarea element represents a multiline plain text edit control. The contents of the control represent the control's default value." but in context of elm, I think the default value is better supplied in other ways.

[galtenberg]

This documentation is helpful (to me, found tonight), but it also potentially leads the user down the path of creating a worse experience when setting value reactively (the cursor will seem to not move as it gets updated with every key press - see #52).

Could align with the sentiment on https://github.com/elm-lang/html/issues/55#issuecomment-233513659 and suggest using defaultValue.