For example, if you learned how to
- configure a router
- backup your server remotely
- process a XML file in your program
If you think that you'll be apart from your task, it's highly recommended that you document your tasks and your software programs and as much as possible provide your sources, they will save your day shortly.
In a recent interview with Donald Knuth I found something interesting. For programming languages, he has something to say. He proposes something called programming literate. It deals with different aspects related with the software development process. It provides guidelines for software documentation, design documentation, agile documentation, and source comments.
I can assure you, a well written document is as important as the product that accompanies it.