Document orphaned datasets/plates retrieval via getProjects/getScreens

OME-DCO-1.1-Signed-off-by: Sebastien Besson <seb.besson@gmail.com> (github: sbesson)
Merge pull request #857 from sbesson/rebased/develop/matlab_orphan

Document orphaned datasets/plates retrieval via getProjects/getScreens (rebased onto develop)