Should I document my unit test methods?

Oh yes!

Even if "who has access to the source code" is never ever going to be anyone else than you, it wont be the same you that look at it in a year (or even a month from now), trust me on that one :-)

Unit tests should aim to be self descriptive, but there will always be cases where this goal cannot be achieved and thus description of what has been written is due.

In other words, try to make your unit tests so they don't need documentation, but if they need it, write it!

I would rather be inclined to saying that you should name your test method in a manner that is expressive with regards to what it tests: SomeMethodWillBehaveThisWayWhenCalledWithTheseParameters() although some people may find that controversial.