Not sure where you would find a document, but an explanation is easy:
There are two types of libraries:
1/ Kicad supplied libraries.
These are read only libraries that cannot be modified. They may be used in the Kicad suite of programs or they may be copied to another location for external saving or modification.
They exist along side the Kicad program in your computer and are regularly upgraded along with the Kicad program.
2/ Everything else.
This consists of all created, imported (from 3rd party), custom and copied and/or modified from Kicad supplied libraries.
These are all placed in Personal Libraries created by you in a place of your choosing that has both read and write attributes.
There is a good (I hope) explanation for Personal Libraries in the FAQ here.
All created, imported (Kicad or 3rd. party) and modified symbols and footprints are addressed through the Symbol and Footprint Editors.
The recognised Kicad workflow is:
1/ Create a Schematic.
Inside the properties of each Symbol on that Schematic, the Footprint should be attached.
2/ Create the PCB from the Schematic.
Inside the properties of each Footprint, the 3D image should be attached.
There are alternatives, but the most trouble free method of creating a project is to start with a Schematic, conclude with the PCB and mechanically verify with the 3D image.
If you have any more questions, please ask. 