Documentation: Remove confusing root-project-name in 'Delegating group' section

Based on introductory section of about page, root project is a project
which inherits from All-Projects. This definition also agrees with
plugin source code.

So, using root-project-name in delegating group is not correct since
this project will be inherited from non All-Projects parent project.

Change-Id: Id26667292b06d7059e1f3b2624ac54bfc5b63d1e