Clarify information about error results
One could (and did) interpret that cqe->res mathes the return *value* of preadv2 and pwritev2.
Clarify that it matches the errno values used by these two.
@@ -944,7 +944,7 @@ They are described in section
.B CQE ERRORS.
.PP
For read and write opcodes, the
-return values match those documented in the
+return values match errno values documented in the
.BR preadv2 (2)
and
.BR pwritev2 (2)
