cgroups.7: Document /sys/kernel/cgroup/features

Reviewed-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/cgroups.7

@@ -1530,6 +1530,31 @@ cgroup.procs
 cgroup.subtree_control
 .in
 .EE
+.TP
+.IR /sys/kernel/cgroup/features " (since Linux 4.15)"
+.\" commit 5f2e673405b742be64e7c3604ed4ed3ac14f35ce
+Over time, the set of cgroups v2 features that are provided by the
+kernel may change or grow,
+or some features may not be enabled by default.
+This file provides a way for user-space applications to discover what
+features the running kernel supports or has enabled.
+Features are listed one per line:
+.IP
+.in +4n
+.EX
+.EE
+$ \fBcat /sys/kernel/cgroup/features\fP
+nsdelegate
+.in
+.IP
+The entries that can appear in this file are:
+.RS
+.TP
+.IR nsdelegate " (since Linux 4.15)"
+The kernel supports the
+.I nsdelegate
+mount option.
+.RE
 .SH ERRORS
 The following errors can occur for
 .BR mount (2):