diff --git a/doc/lispref/hash.texi b/doc/lispref/hash.texi
index a197399..611225c 100644
--- a/doc/lispref/hash.texi
+++ b/doc/lispref/hash.texi
@@ -225,6 +225,27 @@ This function calls @var{function} once for each of the associations in
 @var{table}.  The function @var{function} should accept two
 arguments---a @var{key} listed in @var{table}, and its associated
 @var{value}.  @code{maphash} returns @code{nil}.
+
+Typically, @var{function} will just inspect its arguments.  However,
+it is also allowed to read and/or modify @var{table}, even for
+unrelated keys.  This will not cause @code{maphash} to malfunction,
+but some effects are not fully defined, as described below.
+
+If @var{function} adds a new entry to the @var{table}, it @emph{may or
+may not} be called during this iteration with the new key/value pair.
+
+If @var{function} changes the value already associated with a key
address@hidden and it has not been called with @var{K} yet, it will be called
+with the @var{K} and the @emph{new} value during the iteration later.
+Otherwise it will not be called for @var{K} again, even though it has
+been called with the old value before.
+
+If @var{function} removes a key and it has not been called with it
+yet, it will not be called for the removed key in the future either.
+
+In particular, it follows that if @var{function} removes or changes
+value only for the @var{key} it is called with, the behavior is
+completely defined.
 @end defun
 
 @node Defining Hash
diff --git a/src/fns.c b/src/fns.c
index 825e443..65d46f7 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -4715,7 +4715,10 @@ DEFUN ("remhash", Fremhash, Sremhash, 2, 2, 0,
 DEFUN ("maphash", Fmaphash, Smaphash, 2, 2, 0,
        doc: /* Call FUNCTION for all entries in hash table TABLE.
 FUNCTION is called with two arguments, KEY and VALUE.
-`maphash' always returns nil.  */)
+`maphash' always returns nil.
+
+FUNCTION may modify the TABLE even as iteration is in progress.
+See Info node `(elisp)Hash Access' for details.  */)
   (Lisp_Object function, Lisp_Object table)
 {
   struct Lisp_Hash_Table *h = check_hash_table (table);