gitignore: Add actually generated files, comments

Also remove remnants from automake buildsystem.
edoc for inotify:watch/1 and /2: adds or MODIFIES watch

Make clearer that if a watch already exists on a path, calling
inotify:watch/1 or inotify:watch/2 again on that path will modify
the existing watch instead of generating a new one.

Quoting the inotify_add_watch(2) man page:

    inotify_add_watch() adds  a  new watch, or modifies
    an existing watch, for the file whose location is
    specified in pathname; [...]
Merge pull request #1 from ndim/proposed-changes

Improve watch/n edoc, and clean up .gitignore