Fix for ticket #309 (meaning/definition of project.basedir)

See http://www.phing.info/trac/ticket/309

This might be considered a BC break. Acutally it reverts behaviour to what it was many moons ago.
The fix was (IMO) wrong in the first place, improved docs (including docbook) instead.

(cherry-picking eaf81bc316ed965f4d57b66c75a65e5dbf9e0c8c off unstable-3.0)