label templates: document Random vars are rendered once (#370)

Signed-off-by: Francesco Giudici <[email protected]>

docs/label-templates-random.md

@@ -13,6 +13,19 @@ They allow to include random `Int`, `Hex` or `UUID` values in custom [label temp
 
 The values are computed on the fly during the `label template variables` rendering.
 
+:::info Random label templates are rendered only once
+
+A label template containing a Random variable is rendered only if the
+[MachineInventory](machineinventory-reference) of the registering
+host doesn't have a value for that label yet (a label with the same key is missing or its value is empty).
+
+So, the three cases in which a label template with a Random variable is rendered are:
+1. the host is registering for the first time and the [MachineInventory](machineinventory-reference) is created anew
+2. the label template has been added to the MachineRegistration after the host (re-)registered last time
+3. the [MachineInventory](machineinventory-reference) label matching the label template (same label key) has been manually removed
+or its value has been cleared out
+:::
+
 | Variable                                 | Description                                                           |
 | ---------------------------------------- | --------------------------------------------------------------------- |
 | `${Random/UUID}`                         | random UUID (e.g., fd95324a-c26b-4e28-8727-1dcec293a0ec)              |