To build the documentation:

ant docs

Building the documentation relies on access to a copy of the Grails
source code.  By default the build will download a copy of the source
code from Github.  The archive downloaded will be snaphshot of the latest
version of the project, not the entire project history.  Because of that,
the download is smaller than a full clone of the repository but is still
fairly large (approximately 40mb).  To avoid the download you may take
advantage of a local copy of the source tree by setting the grails.home
system property and assigning it a value that points to a copy of the
grails project:

ant docs -Dgrails.home=/home/user/projects/grails-core