Fix return value and docs for dns_zone_getxfr()

Fix return value and docs for dns_zone_getxfr()

When the given zone is not associated with a zone manager, the function
currently returns ISC_R_NOTFOUND, which is documented as the return
value for the case in which no incoming zone transfer is found.  Make
the function return ISC_R_FAILURE in such a case instead.

Also update the description of the function as the value it returns is
not meant to indicate whether an ongoing incoming transfer for the given
zone exists.  The boolean variables that the function sets via the
pointers provided as its parameters, combined with either keeping
'*xfrp' set to NULL or updating it to a valid pointer, can be used by
the caller to infer all the necessary information.