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>

arch/s390/include/asm/vx-insn-asm.h

@@ -195,10 +195,26 @@
 /* RXB - Compute most significant bit used vector registers
  *
  * @rxb:	Operand to store computed RXB value
- * @v1:		First vector register designated operand
- * @v2:		Second vector register designated operand
- * @v3:		Third vector register designated operand
- * @v4:		Fourth vector register designated operand
+ * @v1:		Vector register designated operand whose MSB is stored in
+ *		RXB bit 0 (instruction bit 36) and whose remaining bits
+ *		are stored in instruction bits 8-11.
+ * @v2:		Vector register designated operand whose MSB is stored in
+ *		RXB bit 1 (instruction bit 37) and whose remaining bits
+ *		are stored in instruction bits 12-15.
+ * @v3:		Vector register designated operand whose MSB is stored in
+ *		RXB bit 2 (instruction bit 38) and whose remaining bits
+ *		are stored in instruction bits 16-19.
+ * @v4:		Vector register designated operand whose MSB is stored in
+ *		RXB bit 3 (instruction bit 39) and whose remaining bits
+ *		are stored in instruction bits 32-35.
+ *
+ * Note: In most vector instruction formats [1] V1, V2, V3, and V4 directly
+ * correspond to @v1, @v2, @v3, and @v4. But there are exceptions, such as but
+ * not limited to the vector instruction formats VRR-g, VRR-h, VRS-a, VRS-d,
+ * and VSI.
+ *
+ * [1] IBM z/Architecture Principles of Operation, chapter "Program
+ * Execution, section "Instructions", subsection "Instruction Formats".
  */
 .macro	RXB	rxb v1 v2=0 v3=0 v4=0
 	\rxb = 0
@@ -223,6 +239,9 @@
  * @v2:		Second vector register designated operand (for RXB)
  * @v3:		Third vector register designated operand (for RXB)
  * @v4:		Fourth vector register designated operand (for RXB)
+ *
+ * Note: For @v1, @v2, @v3, and @v4 also refer to the RXB macro
+ * description for further details.
  */
 .macro	MRXB	m v1 v2=0 v3=0 v4=0
 	rxb = 0
@@ -238,6 +257,9 @@
  * @v2:		Second vector register designated operand (for RXB)
  * @v3:		Third vector register designated operand (for RXB)
  * @v4:		Fourth vector register designated operand (for RXB)
+ *
+ * Note: For @v1, @v2, @v3, and @v4 also refer to the RXB macro
+ * description for further details.
  */
 .macro	MRXBOPC	m opc v1 v2=0 v3=0 v4=0
 	MRXB	\m, \v1, \v2, \v3, \v4