Update ULN2003.md

Added a couple more sentences about the COM pin - hopefully that would have prevented the confusion about it i saw in that thread. 

Also added a Caveats section to spare everyone the pain I went through when I discovered that "500ma/channel" didn't mean "500ma/channel, at up to 100% duty cycle on each of the 7 channels". 

Added Variants section to mention a few similar parts.