Add a bit of documentation for the various undocumented environment variables we honour

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+# Known Environment Variables
+
+A number of systemd components take additional runtime parameters via
+environment variables. Many of these environment variables are not supported at
+the same level as command line switches and other interfaces are: we don't
+document them in the man pages and we make no stability guarantees for
+them. While they generally are unlikely to be dropped any time soon again, we
+do not want to guarantee that they stay around for good either.
+
+Below is an (incomprehensive) list of the environment variables understood by
+the various tools. Note that this list only covers environment variables not
+documented in the proper man pages.
+
+All tools:
+
+* `$SYSTEMD_IGNORE_CHROOT=1` — if set, don't check whether being invoked in a
+  chroot() environment. This is particularly relevant for systemctl, as it will
+  not alter its behaviour for chroot() environments if set. (Normally it
+  refrains from talking to PID 1 in such a case.)
+
+* `$SD_EVENT_PROFILE_DELAYS=1` — if set, the sd-event event loop implementation
+  will print latency information at runtime.
+
+systemctl:
+
+* `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus
+  listener, and instead always connect through the dbus-daemon D-bus broker.
+
+* `$SYSTEMCTL_INSTALL_CLIENT_SIDE=1` — if set, enable or disable unit files on
+  the client side, instead of asking PID 1 to do this.
+
+* `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call out to SysV compatibility hooks.
+
+systemd-nspawn:
+
+* `$UNIFIED_CGROUP_HIERARCHY=1` — if set, force nspawn into unified cgroup
+  hierarchy mode.
+
+* `$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1` — if set, make /sys and /proc/sys and
+  friends writable in the container. If set to "network", leave only
+  /proc/sys/net writable.
+
+* `$SYSTEMD_NSPAWN_CONTAINER_SERVICE=…` — override the "service" name nspawn
+  uses to register with machined. If unset defaults to "nspawn", but with this
+  variable may be set to any other value.
+
+* `$SYSTEMD_NSPAWN_USE_CGNS=0` — if set, do not use cgroup namespacing, even if
+  it is available.
+
+* `$SYSTEMD_NSPAWN_LOCK=0` — if set, do not lock container images when running.
+
+systemd-logind:
+
+* `$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1` — if set, report that
+  hibernation is available even if the swap devices do not provide enough room
+  for it.