Update REST docs to match changes in GEOS-5512. (2.3.x)

86d8ed7 added a workspaces/ container around REST urls for service-specific settings.

Doesn't appear the doc changes around restconfig -> rest included it, so the current rest api docs don't refer to the container.

This PR is for 2.3.x, see https://github.com/geoserver/geoserver/pull/302 for master, and https://github.com/geoserver/geoserver/pull/303 for 2.4.x