membarrier.2: Clarify return value for MEMBARRIER_CMD_QUERY

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/membarrier.2

@@ -126,7 +126,11 @@ The pair ordering is detailed as (O: ordered, X: not ordered):
        sys_membarrier()   O          O          O
 
 .SH RETURN VALUE
-On success, this system call returns zero.
+On success, the
+.B MEMBARRIER_CMD_QUERY
+operation returns a bit mask of supported commands and the
+.B MEMBARRIER_CMD_SHARED
+operation returns zero.
 On error, \-1 is returned,
 and
 .I errno