As a suggestion, I think it would really help consolidate common shape, line, and layout into its own section in the help. I find it hard to find (and therefore understand) general rules like these for platuml, they seem to be spread across lots of diagram speciifc pages.
For example "left to right" is documented under use cases. HTML annotation and link notes is under "Class Diagram". Labels on arrows is under Activity Diagram. And I am not sure where arrow length (how it affects layout) is, but it sure is useful to know :).
Having all that stuff on one page I think would be more helpful for users. That page could talk about layout tricks like hidden arrows as well. I know there is Common Commands, but it doesn't touch layout items like the above.