Document limitation on caml_callbackN (#11409)

The `args` argument array must not be declared with `CAMLlocalN`, otherwise
the array can be registered a second time by `caml_callbackN`, confusing
the compactor.

Also: suggests using a C99 compound literal, it looks good and avoids the issue.

Fixes: #11045

Changes

@@ -25,6 +25,13 @@ OCaml 4.14 maintenance branch
 - #11392, #11392: assertion failure with -rectypes and external definitions
   (Gabriel Scherer, review by Florian Angeletti, report by Dmitrii Kosarev)
 
+### Manual and documentation:
+
+- #11045, #11409: document that the array argument to `caml_callbackN`
+  must not have been declared by `CAMLlocalN`.
+  (Xavier Leroy, report by Stephen Dolan, review by Gabriel Scherer.)
+
+
 OCaml 4.14.0 (28 March 2022)
 ----------------------------
 

manual/src/cmds/intf-c.etex

@@ -1449,6 +1449,10 @@ the value \var{a} and returns the value returned by~\var{f}.
 (a curried OCaml function with three arguments) to \var{a}, \var{b} and \var{c}.
 \item "caml_callbackN("\var{f, n, args}")" applies the functional value \var{f}
 to the \var{n} arguments contained in the C array of values \var{args}.
+The array \var{args} must \emph{not} be declared with "CAMLlocalN".
+It should be declared as "value "\var{args}"["\var{n}"];".
+Alternatively, a C99 compound literal can be used:
+"caml_callbackN("\var{f, n, }"(value[]){"\nth{arg}{1}, \ldots, \nth{arg}{n}"})".
 \end{itemize}
 If the function \var{f} does not return, but raises an exception that
 escapes the scope of the application, then this exception is