Clarify permitted mutation in `maphash` documentation

* doc/lispref/hash.texi (Hash Access):
* src/fns.c (Fmaphash):
Make it clear what the function passed as argument can do.  Until now
these rules were unwritten, and are still unenforced.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/fns.c b/src/fns.c
index 4531b237824..f862c1470c4 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -5651,6 +5651,8 @@ 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.
+It should not alter TABLE in any way other than using `puthash' to
+set a new value for KEY, or `remhash' to remove KEY.
 `maphash' always returns nil.  */)
   (Lisp_Object function, Lisp_Object table)
 {