envsetup.sh: Update text in lunch menu for clarity

Updates the lunch menu text to make it clearer that the choices in lunch
menu are not fully comprehensive. As the list comes from common lunch
choices the text now refers to 'common combinations' and informs that
you can 'specify your own'. This should help developers who are unsure
why they cannot find their desired lunch target in the list.

Change-Id: Ib9d55ee46b7037c30a8f4b43fdd552ea66e887e8
Test: source build/envsetup.sh && lunch