Documentation helps developers use your components to develop their apps more effectively. You can provide interactive examples, documentation, and specification descriptions for
- Component
- Event
- Interface.
Each component, event, or interface has a root definition that defines the element’s metadata, as well as attributes, events, or methods.
Component – A component’s root definition is specified in the <aura:component> tag contained in componentName.cmp.
Event – An event’s root definition is specified in the <aura:event> tag contained in componentEvent.evt.
Interface – An interface’s root definition is specified in the <aura:interface> tag contained in the interfaceName.intf.
The documents that you create will be available in.
https://<domain>.lightning.force.com/auradocs/reference.app

Where <Domanin_Name> is the name of custom salesforce domain that is created.
You can access component liberary as.
https://<domain>.lightning.force.com/docs/component-library/overview/components

Where <Domanin_Name> is the name of custom salesforce domain that is created.
Note:
A Documentation is in lightning bundle is saved as componentName.auradoc. If you component name is My_Lightning_Component, then a controller will be saved as My_Lightning_Component.auradoc.