Improve doc for .container-smooth

[fazliddin]

.container-smooth can not be used along. It must be added to container div with container-fluid class, that is, <div class="container-fluid container-smooth">. Because bootstrap container classes has useful properties:

.container-fluid {
    padding-right: 15px;
    padding-left: 15px;
    margin-right: auto;
    margin-left: auto;
}

Without them horizontal ruler appears. Except that container* classes used to identify other classes. For example, .jumbotron class loses properties if used outside container or container-fluid