Document Schema 'description' fields in code documentation

This patch adds documentation for the 'description' field to the Schema API code documentation. Nothing but the @defgroup schemaapi comment is affected. It is important that this go in before the beta release so module others that check api.d.o while writing their .schema files see it.

This patch is different than http://drupal.org/node/164983 in which we are actually adding description fields to the core schema structures. This patch merely documents that the description fields exist.

I thought I submitted this patch ages ago but apparently I forgot. Sorry for the rush.