build: 'sync-descriptions' now compares GNU package descriptions.

* build-aux/sync-synopses.scm: Rename to...
* build-aux/sync-descriptions.scm: ... this.  Compare the value of
  'gnu-package-doc-description' and 'package-description' and report any
  mismatch.
* Makefile.am (EXTRA_DIST): Adjust accordingly.
  (sync-synopses): Rename to...
  (sync-descriptions): ... this.