manifest_xml: harmonize list fields

We allow project.groups to be whitespace or comma delimited, but
repo-hooks.enabled-list is only whitespace delimited.  This hasn't
been a big deal as it's only ever had one valid value, but if we
want to add more, we should harmonize these a bit.

Refactor the groups method to be more generic, and run the enabled-
list attribute through it.  Then add missing docs for it.

Change-Id: Iaa96a0faa9c4a68b313b49336751831b73bf855d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/290743
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

docs/manifest-format.md

@@ -111,6 +111,10 @@ A description of the elements and their attributes follows.
 
 The root element of the file.
 
+### Element notice
+
+Arbitrary text that is displayed to users whenever `repo sync` finishes.
+The content is simply passed through as it exists in the manifest.
 
 ### Element remote
 
@@ -360,6 +364,19 @@ This element is mostly useful in a local manifest file, where
 the user can remove a project, and possibly replace it with their
 own definition.
 
+### Element repo-hooks
+
+NB: See the [practical documentation](./repo-hooks.md) for using repo hooks.
+
+Only one repo-hooks element may be specified at a time.
+Attempting to redefine it will fail to parse.
+
+Attribute `in-project`: The project where the hooks are defined.  The value
+must match the `name` attribute (**not** the `path` attribute) of a previously
+defined `project` element.
+
+Attribute `enabled-list`: List of hooks to use, whitespace or comma separated.
+
 ### Element include
 
 This element provides the capability of including another manifest