The parser treats (R1+R2) on ppc64 the same as (R1,R2) on arm,
but it is not strictly a "register pair". Improve the text.

No semantic change.

Change-Id: Ib8b14881c6467add0d53150a901c01e962afb28b
Reviewed-on: https://go-review.googlesource.com/12212Reviewed-by: Russ Cox <rsc@golang.org>