* src/xdisp.c (vmessage, message): Clarify commentary.

src/xdisp.c

@@ -10470,9 +10470,12 @@ message_with_string (const char *m, Lisp_Object string, bool log)
 /* Dump an informative message to the minibuf.  If M is 0, clear out
    any existing message, and let the mini-buffer text show through.
 
-   The message must be safe ASCII and the format must not contain ` or
-   '.  If your message and format do not fit into this category,
-   convert your arguments to Lisp objects and use Fmessage instead.  */
+   The message must be safe ASCII (because when Emacs is
+   non-interactive the message is sent straight to stderr without
+   encoding first) and the format must not contain ` or ' (because
+   this function does not account for `text-quoting-style').  If your
+   message and format do not fit into this category, convert your
+   arguments to Lisp objects and use Fmessage instead.  */
 
 static void ATTRIBUTE_FORMAT_PRINTF (1, 0)
 vmessage (const char *m, va_list ap)
@@ -10530,6 +10533,7 @@ vmessage (const char *m, va_list ap)
     }
 }
 
+/* See vmessage for restrictions on the text of the message.  */
 void
 message (const char *m, ...)
 {