Must: document that link walking is incompatible with security

[macintux]

Probably worth throwing this into both authz.md and the link walking document. Also the 2.0 overview doc.

            %% check for link walk queries when security is enabled,
            %% we don't allow them.
            case {riak_core_security:is_enabled(),
                  lists:keyfind(link, 1, Query)} of
                {true, Links} when Links /= false ->
                    %% we don't support link walking when security is
                    %% enabled, return an error of some kind
                    {error, {format, "Link walking is"
                             " deprecated in Riak 2.0 and is"
                             " not compatible with security."}, State};