Sharp/soft knee refers to how a device works in term of the ratio 'kicking in', I wouldn't say it has anything to do with the comp/limit thing. On a standard (harman) DBX compressor, there is the hard mode or the soft knee mode. The difference is that once the signal has reached the threshold, in the hard mode it will be reduced strictly by the ratio, whereas in the soft mode, it will have a certain transition region before the ratio actually kicks in...as in it comes in gradually rather than abruptly. I don't have a link to anything, but this is easy to look at graphically. Any good ole recording handbook should have pix of this type of stuff, and also comps with limiters that kick in at a higher ratio.
I really don't think there is any point in being too strict about the limiter/compressor definition. I'm sure PRR could chime in with lots of history, but remember that we are dealing with devices that evolved over a long period of time. A limiter like the LA2A is not fast by today's standards, and many of the old comps that we all lust after have variable ratios anyway, depending on how far over threshold you push them. As far as sound goes, you can't really say something sounds like a limiter or a compressor, it's all a bit blurry!
Bjorn
...although as Thomas says, yes you certainly would expect a modern limiter to kick in with a hard ratio very quickly to make sure nothing is let through past whatever point you've set.