All of the CRI store related packages all use the standard errdefs
errors now for if a key doesn't or already exists (ErrAlreadyExists,
ErrNotFound), but the comments for the methods still referenced
some unused package specific error definitions. This change just
updates the comments to reflect what errors are actually returned
and adds comments for some previously undocumented exported functions.
Signed-off-by: Daniel Canter <dcanter@microsoft.com>