Update documentation for building Autoconf from a Git checkout.

* Clarify that you need a full Git clone, not a shallow clone, because
  the build scripts inspect the Git history.

* Remove all references to CVS and the CVS mirror of the Git repo.
  pserver.git.sv.gnu.org doesn’t exist anymore, and I suspect building
  from a CVS-based working copy wouldn’t work anyway, for the same
  reason a shallow clone doesn’t work.

* Add a note to the paragraph about copyright assignments for patches,
  explaining that that Signed-off-by: is useless to us.