CFE-4686: Document the classes promise cancel attribute

content/reference/promise-types/classes.markdown

@@ -100,6 +100,60 @@ If an expression contains a mixture of different object types that need to be
 ANDed together, this list form is more convenient than providing an
 expression.
 
+### cancel
+
+**Description:** Undefine the promiser class when this string expression of
+classes evaluates to true
+
+Undefine the class on the left-hand side if the class expression on the
+right-hand side evaluates to true. This is the inverse of
+[`expression`][classes#expression]: where `expression` defines a class,
+`cancel` removes one that is already defined.
+
+The right-hand side is an ordinary class expression, so it may be a class name,
+a compound expression, or the return value of a function that returns a class,
+such as `fileexists()`.
+
+Unlike the class-defining attributes, a `cancel` promise is evaluated **even
+when the promiser class is already defined** — that is precisely when it has
+work to do. If the cancel expression is true the class is undefined. If the
+expression is false, or the class is not currently defined, the promise makes
+no change. Reserved hard classes (such as `linux` or `cfengine`) cannot be
+undefined and are left in place with a warning.
+
+`cancel` is mutually exclusive with the class-defining attributes (`expression`,
+`and`, `or`, `not`, `xor`, `dist`, `select_class`); combining it with any of
+them is an `Irreconcilable constraints` error.
+
+**Type:** `class`
+
+**Allowed input range:** `[a-zA-Z0-9_!@@$|.()\[\]{}:]+`
+
+**Example:**
+
+```cf3 {skip TODO}
+classes:
+
+    # Undefine 'maintenance' once the maintenance window has passed
+    "maintenance" cancel => "maintenance_window_over";
+
+    # Undefine 'have_config' if the file no longer exists
+    "have_config" cancel => not(fileexists("/etc/myapp.conf"));
+```
+
+**Notes:**
+
+A class may be defined again later in the same agent execution (for example by
+a class-defining promise evaluated in a later pass). `cancel` therefore does not
+guarantee that a class stays undefined for the remainder of the run; it
+undefines the class at the point the promise is evaluated.
+
+**History:**
+
+- Introduced in CFEngine 3.28.0.
+
+**See also:** [`expression` classes attribute][classes#expression], [`cancel_kept`, `cancel_repaired` and `cancel_notkept` in classes body][Promise types#cancel_kept]
+
 ### dist
 
 **Description:** Generate a probabilistic class distribution