Currently by the end of the page https://www.atlassian.com/software/sourcetree/overview there is a link that leads to a SourceTree Documentation. This link currently takes to the SourceTree Knowledge Base.

Some users would like to have a manual/tutorial that guides at least through the basics of SourceTree.

I've found some useful information in this Answers topic, which mentions a git tutorial (http://www.atlassian.com/git) that may should be referenced in the documentation, as well as a "SourceTree Help" that is included in the Mac version.

Summarizing, the suggestion is basically to have a documentation exposing SourceTree functionality and basic steps.