Well,
Short answer is:
If you want to document an algorithm (that is used by one of your system object^s method): Use Activity diagrams.
Long Answer:
For my experience UML documentations are the worst ones.
People generally use Uml Tools which automaticly reverse engineer code to Uml (generally class diagrams, and sometimes sequence diagrams) and diagrams that are automatically generated are generally has many details and sometimes nonsense.
As an advice "Comments your code properly" and use tools like Doxygen. They are better for Code Documentation.
But you can use UML for Software Architect Document.[SAD]. Craig Larman has nice section and example of it at his books about Documenting Architecture
Motivation: Why Create a SAD?
When someone joins the development team, it's useful if the project coach can say, "Welcome to the NextGen project! Please go to the project website and read the ten page SAD in order to get an introduction to the big ideas." And later, during a subsequent release, when new people work on the system, a SAD can be a learning aid to speed their comprehension.
Therefore, it should be written with
this audience and goal in mind: What
do I need to say (and draw in the UML)
that will quickly help someone
understand the major ideas in this
system? [ Applying UML and Patterns Third Edition By Craig Larman ] [Chapter 39. Documenting Architecture: UML & the N+1 View Model]