DocBook: Add a help message for DOCBOOKS env var

Sometimes, it is needed to compile only a subset of the possible
DocBooks. This is supported by the building system, but it is not
docummented. Add a documentation for it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>