s390/fpu: improve description of RXB macro

The v1, v2, v3, and v4 parameters of the RXB macro are a bit misleading,
since the reader can assume that the parameters always correlate with the
instructions format fields V1, V2, V3, and V4 as defined in the Principles
of Operation.

This is not the case for a couple of instructions, therefore improve the
description of the macro.

Suggested by Jens Remus, who also provided the improved description.

Suggested-by: Jens Remus <jremus@linux.ibm.com>
Reviewed-by: Jens Remus <jremus@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>