Improve the descriptions of the example Managed Bookmarks extension.

This example extension manages the bookmarks via configuration policy. This
change improves the policy schema by adding more optional documentation, so
that configuration tools can show human-readable help, and by having a more
user-friendly description for the extension, since it will be eventually
published.

BUG=108992

Review URL: https://codereview.chromium.org/100603005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239476 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239476