gnu-maintenance: Add `doc-description' field to <gnu-package-descriptor>.

* guix/gnu-maintenance.scm (%gsrc-package-list-url): New variable.
  (<gnu-package-descriptor>): Add `doc-description' field.
  (official-gnu-packages)[group-package-fields]: Rename to...
  [read-records]: ... this.  Reverse the result.
  [gsrc-description]: New procedure.
  Add the "description" field to the alist passed to `alist->record'.