[machineid] Improve documentation on this module

- document module's purpose
 - document existing configurations
 - deprecate "symlink" and introduce more-accurate "dbus-symlink"
 - add new configurations for upcoming entropy file

src/modules/machineid/machineid.conf

@@ -1,8 +1,24 @@
+# Machine-ID and other random data on the target system.
+#
+# This module can create a number of "random" things on the target:
+#  - a systemd machine-id file (hence the name of the Calamares module)
+#    with a random UUID.
+#  - a dbus machine-id file (or, optionally, link to the one from systemd)
+#  - an entropy file
+#
 ---
 # Whether to create /etc/machine-id for systemd.
 systemd: true
+
 # Whether to create /var/lib/dbus/machine-id for D-Bus.
 dbus: true
 # Whether /var/lib/dbus/machine-id should be a symlink to /etc/machine-id
 # (ignored if dbus is false, or if there is no /etc/machine-id to point to).
+dbus-symlink: true
+# this is a deprecated form of *dbus-symlink*
 symlink: true
+
+# Whether to create an entropy file
+entropy: false
+# Whether to copy entropy from the host
+entropy-copy: false