Help: Expand documentation of variable_watch()

Document parameters and corner-case behaviour of variable_watch() command.

Help/command/variable_watch.rst

@@ -7,9 +7,42 @@ Watch the CMake variable for change.
 
   variable_watch(<variable> [<command>])
 
-If the specified ``<variable>`` changes, a message will be printed
-to inform about the change.
+If the specified ``<variable>`` changes and no ``<command>`` is given,
+a message will be printed to inform about the change.
 
-Additionally, if ``<command>`` is given, this command will be executed.
+If ``<command>`` is given, this command will be executed instead.
 The command will receive the following arguments:
 ``COMMAND(<variable> <access> <value> <current_list_file> <stack>)``
+
+``<variable>``
+ Name of the variable being accessed.
+
+``<access>``
+ One of ``READ_ACCESS``, ``UNKNOWN_READ_ACCESS``, ``MODIFIED_ACCESS``,
+ ``UNKNOWN_MODIFIED_ACCESS``, or ``REMOVED_ACCESS``.  The ``UNKNOWN_``
+ values are only used when the variable has never been set.  Once set,
+ they are never used again during the same CMake run, even if the
+ variable is later unset.
+
+``<value>``
+ The value of the variable.  On a modification, this is the new
+ (modified) value of the variable.  On removal, the value is empty.
+
+``<current_list_file>``
+ Full path to the file doing the access.
+
+``<stack>``
+ List of absolute paths of all files currently on the stack of file
+ inclusion, with the bottom-most file first and the currently
+ processed file (that is, ``current_list_file``) last.
+
+Note that for some accesses such as :command:`list(APPEND)`, the watcher
+is executed twice, first with a read access and then with a write one.
+Also note that an :command:`if(DEFINED)` query on the variable does not
+register as an access and the watcher is not executed.
+
+Only non-cache variables can be watched using this command.  Access to
+cache variables is never watched.  However, the existence of a cache
+variable ``var`` causes accesses to the non-cache variable ``var`` to
+not use the ``UNKNOWN_`` prefix, even if a non-cache variable ``var``
+has never existed.