gnu: Synchronize synopses and descriptions with the Womb.

* gnu/packages/gnunet.scm (gnunet): Synchronize description and synopsis
  with upstream.
* gnu/packages/maths.scm (units): Likewise.
* gnu/packages/grub.scm (grub): Likewise.
* gnu/packages/pretty-print.scm (enscript): Likewise.