Clarify that the optional '-' prefix of the 'tentative', 'deprecated'
and 'dadfailed' keywords has to be put right in front of them, no
whitespace is allowed in between.

In addition to that, clarify that it is valid to pass both 'valid_lft'
and 'preferred_lft' at the same time to 'ip address'.
Signed-off-by: Phil Sutter <phil@nwl.cc>