Improve watch/n edoc, and clean up .gitignore

While playing around with __inotify__, I ran into the issue that adding a second watch for the same dir is not possible -- so I documented that fact in the `watch/1` and `watch/2` functions.

Also, the `.gitignore` file needed a bit of maintenance, so I removed some files not generated any more, and added some comments for the remaining files and the ones I added.