setns.2: Some logical text restructuring

Add subsection headings and use hanging list for details
of specific namespace types.

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

man2/setns.2

@@ -99,7 +99,12 @@ is a descendant (child, grandchild, etc.)
 of the PID namespace of the caller.
 For further details on PID namespaces, see
 .BR pid_namespaces (7).
-.PP
+.\"
+.SS Details for specific namespace types
+Note the following details and restrictions when reassociating with
+specific namespace types:
+.TP
+User namespaces
 A process reassociating itself with a user namespace must have the
 .B CAP_SYS_ADMIN
 .\" See kernel/user_namespace.c:userns_install() [3.8 source]
@@ -126,7 +131,8 @@ filesystem-related attributes
 flag) with another process.
 For further details on user namespaces, see
 .BR user_namespaces (7).
-.PP
+.TP
+Mount namespaces
 A process may not be reassociated with a new mount namespace if it is
 multithreaded.
 .\" Above check is in fs/namespace.c:mntns_install() [3.8 source]
@@ -140,7 +146,8 @@ in the the user namespace that owns the target mount namespace.
 See
 .BR user_namespaces (7)
 for details on the interaction of user namespaces and mount namespaces.
-.PP
+.TP
+Cgroup namespaces
 Using
 .BR setns ()
 to change the caller's cgroup namespace does not change