Component documentation helps others understand and use your components. Here is a sample example of Lightning Component Documentation.

Sample Component:

<!--Sample.cmp--> 
<aura:component implements="flexipage:availableForAllPageTypes" access="global">
    <strong>Sample Component</strong>
</aura:component>

To create documentation of above component, click DOCUMENTATION in the component sidebar of the Developer Console. Hereaura:description is to show the description of the component and aura:example is to show look and feel of the component.

<aura:documentation>
    <aura:description>
        <p>An <code>c:Sample</code> component represents an element that executes an action defined by a controller.</p>
    </aura:description>
    <aura:example name="Sample" ref="c:Sample" label="Using the c:Sample Component">
        <p>This example shows a simple setup of <code>Sample</code>.</p>
    </aura:example>
</aura:documentation>

The documentation you create will be available at https://.lightning.force.com/auradocs/reference.app, where is the name of your custom Salesforce domain and in the Component Library at https://.lightning.force.com/componentReference/suite.app where is the name of your custom Salesforce domain.