mount_setattr.2: Rework the discussion of MOUNT_ATTR__ATIME

Phrases such as "In the new mount API" will date fast. Remove it.
Also:
* Make it clear that MOUNT_ATTR__ATIME expresses a bit field.
* Replace 'enum' with 'enumeration'.
* Clarify what is meant by "partially" set MOUNT_ATTR__ATIME.

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

man2/mount_setattr.2

@@ -270,8 +270,11 @@ and is implied by the noatime setting.
 All other access-time settings are mutually exclusive.
 .TP
 .BR MOUNT_ATTR__ATIME " - changing access-time settings"
-In the new mount API, the access-time values are an enum starting from 0.
-Even though they are an enum (in contrast to the other mount flags such as
+The access-time values listed below are an enumeration that
+includes the value zero, expressed in the bits defined by the mask
+.BR MOUNT_ATTR__ATIME .
+Even though these bits are an enumeration
+(in contrast to the other mount flags such as
 .BR MOUNT_ATTR_NOEXEC ),
 they are nonetheless passed in
 .I attr_set
@@ -282,12 +285,11 @@ for consistency with
 which introduced this behavior.
 .IP
 Note that,
-since access times are an enum
-not a bit map,
-users wanting to transition to a different access-time setting cannot simply
-specify the access-time setting in
-.I attr_set
-but must also set
+since the access-time values are an enumeration rather than bit values,
+a caller wanting to transition to a different access-time setting
+cannot simply specify the access-time setting in
+.IR attr_set ,
+but must also include
 .B MOUNT_ATTR__ATIME
 in the
 .I attr_clr
@@ -295,8 +297,10 @@ field.
 The kernel will verify that
 .B MOUNT_ATTR__ATIME
 isn't partially set in
-.IR attr_clr ,
-and that
+.IR attr_clr
+(i.e., either all bits in the
+.B MOUNT_ATTR__ATIME
+bit field are either set or clear), and that
 .I attr_set
 doesn't have any access-time bits set if
 .B MOUNT_ATTR__ATIME