If you look at https://github.com/moodle/moodle/tree/master/pix/i you can see that there are 8 different completion- icons. However, I cannot find any documentation that explains what they mean. (Look at the pages linked from http://docs.moodle.org/22/en/Category:Completion, for example.)
Just thought I should point that out. Would a MDLSITE bug be better?