Fix broken phrasing.
.gitignore cleanups

* Clarified bogosity of toplevel .gitignore (and other such files)

* Removed "doc/" in the toplevel file -- looks like this was some broken
  leftover, and it's dangerous for at least "scribble/doc" (which
  contains code).

* Collapse more common patterns into worksp instead of in each file.

* Use a trailing "/" for directories.

* racket/lib/collects/pkg/.gitignore looks like a leftover that is no
  longer needed.