Allow projects to be specified as notdefault

Instead of every group being in the group "default", every project
is now in the group "all".   A group that should not be downloaded
by default may be added to the group "notdefault".

This allows all group names to be positive (instead of removing groups
directly in the manifest with -default) and offers a clear way of
selecting every project (--groups all).

Change-Id: I99cd70309adb1f8460db3bbc6eff46bdcd22256f
diff --git a/docs/manifest-format.txt b/docs/manifest-format.txt
index 338e021..f499868 100644
--- a/docs/manifest-format.txt
+++ b/docs/manifest-format.txt
@@ -184,11 +184,12 @@
 
 Attribute `groups`: List of groups to which this project belongs,
 whitespace or comma separated.  All projects belong to the group
-"default", and each project automatically belongs to a group of
-it's name:`name` and path:`path`.  E.g. for
+"all", and each project automatically belongs to a group of
+its name:`name` and path:`path`.  E.g. for
 <project name="monkeys" path="barrel-of"/>, that project
 definition is implicitly in the following manifest groups:
-default, name:monkeys, and path:barrel-of.
+default, name:monkeys, and path:barrel-of.  If you place a project in the
+group "notdefault", it will not be automatically downloaded by repo.
 
 Element annotation
 ------------------