Documentation can take many forms. It can be a simple sheet of what the item is, how it compares to a period model for that item, and a couple of sources for your information. If you are entering Kingdom Arts and Sciences it will expand beyond that.
Don’t forget what the purpose of documentation, to show that you have an understanding of the topic in line with your skill level. As historical arts and sciences is ever changing, you have the opportunity to introduce new ideas to the community. If you are working at an advanced level, the purpose will also be to introduce readers, and judges to new research on a topic and to convey enough information to justify what you have done with your piece as it relates to the period model.
If your entry is a paper, or supported research entry, where your documentation will be due a week before most competitions, feel free to write as much or as little as you feel you need to in order to cover the topic. Note that most judges are looking for academic sources more than those from popular histories, though there can be great information in those as well. Try to balance between the two if you are using both. Try to avoid sites that consolidate information into summaries, such as Wikipedia, Mashable, and zines. However, their bibliographies may be a great way to find the sources you need.
For static entries, keep in mind that your documentation it needs to be relatively concise. Often your judge will have limited reading time. Feel free to include any charts, pictures and glossaries in an appendix. That way, if something needs to be clarified it can be. Take a look at the sample document under competition documentation best practices. Make sure you highlight any new findings in your field. This will make for a better competition experience, and help your judges judge better, as no one can know everything. We all love knowing new information about our passions.