Bootstrap块级帮助文本
Bootstrap表单控件可以具有与输入一起流动的块级帮助文本。要添加全宽内容块，请在<input>之后使用.help-block。
以下示例说明了这一点
示例
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap Example</title>
<link href = "/bootstrap/css/bootstrap.min.css" rel = "stylesheet">
<script src = "/scripts/jquery.min.js"></script>
<script src = "/bootstrap/js/bootstrap.min.js"></script>
</head>
<body>
<form role = "form">
<span>Example of Help Text</span>
<input class = "form-control" type = "text" placeholder = "">
<span class = "help-block">
A longer block of help text that breaks onto a new line and may extend beyond one line.
</span>
</form>
</body>
</html>