Filter documentation improvements

[hardbyte]

Needs a bit more explanation and examples.

Links:

• http://www.cse.dmu.ac.uk/~eg/tele/CanbusIDandMask.html

[felixdivo]

Doesn't this suffice?

[hardbyte]

It is certainly better than it was but still no, not really. It serves as a good reference for those familiar with the library. If I recall someone asked for some examples of using can filters - including what happens to example messages.

More generally python-can could really do with a few documentation guides to compliment the example code and reference docs.