Place holder support At other Areas

## Is your enhancement request related to a problem? Please describe

field Place holder is a good place for educating your user in how to use product. Currently are placeholder for question tile. Request to add other placeholders in following order of priority

1. Question body (I can educate user how to write good question)
2. Answer Body (I can educate user how to write good answer)
3. comment body (least priority - I can ask user to be polite, be respectfull etc)

## Describe the solution you'd like

A clear and concise description of what you want to happen.

## Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

まず、質問タイル用の既存のプレースホルダーを見つけ、そのテキストがどのように設定され、レンダリングされるかをたどります。同じフローを、要求された優先順位に従って質問本文、回答本文、コメント本文にも拡張し、その後、各エディターに設定されたプレースホルダーが表示されることを確認します。

索引モデルが issue の本文から書いたものです。