Document behavior of certain TreeList methods to avoid confusion

[Robotgiggle]

Describe the feature

The new TreeList class provides the following access methods:

• head() gets the first element of the list
• tail() gets the whole list except the first element
• last() gets the last element of the list
• init() gets the whole list except the last element

While apparently standard for these operations, some of these method names (particularly init()) are a little confusing. I would suggest adding javadoc comments explaining what each of them do.

@s5bug