HAFAS: more detailed new_p rejection documentation

=item my $status_p = Travel::Status::DE::HAFAS->new_p(I<%opt>)

Return a promise that resolves into a Travel::Status::DE::HAFAS instance

($status) on success and rejects with an error message and optional HAFAS

instance ($status->errstr[, $status]) on failure. The optional HAFAS instance

is only present if the error occured late, e.g. due to an ambiguous location

specifier.  In addition to the arguments of B<new>, the following mandatory

arguments must be set.

($status) on success and rejects with an error message on failure. If the

failure occured after receiving a response from the HAFAS backend, the rejected

promise contains a Travel::Status::DE::HAFAS instance as a second argument.

This instance can be used e.g. to call similar_stops in case of an ambiguous

location specifier. In addition to the arguments of B<new>, the following

mandatory arguments must be set.

=over