1 files changed, 9 insertions, 0 deletions

diff --git a/src/data.c b/src/data.c
index 81846b5dcbc..eee2a52a37a 100644
--- a/src/data.c
+++ b/src/data.c
@@ -1428,6 +1428,15 @@ harmonize_variable_watchers (Lisp_Object alias, Lisp_Object base_variable)
 DEFUN ("add-variable-watcher", Fadd_variable_watcher, Sadd_variable_watcher,
        2, 2, 0,
        doc: /* Cause WATCH-FUNCTION to be called when SYMBOL is set.
+
+It will be called with 4 arguments: (SYMBOL NEWVAL OPERATION WHERE).
+SYMBOL is the variable being changed.
+NEWVAL is the value it will be changed to.
+OPERATION is a symbol representing the kind of change, one of: `set',
+`let', `unlet', `makunbound', and `defvaralias'.
+WHERE is a buffer if the buffer-local value of the variable being
+changed, nil otherwise.
+
 All writes to aliases of SYMBOL will call WATCH-FUNCTION too.  */)
   (Lisp_Object symbol, Lisp_Object watch_function)
 {