org-html.el: Enhance docstrings

* org-html.el (org-export-html-preamble-format)
(org-export-html-postamble-format): Mention that
org-export-html-preamble/postamble need to be `t' in order for
these variables to have any effect.

lisp/org-html.el

@@ -464,6 +464,9 @@ precedence over this variable."
 (defcustom org-export-html-preamble-format '(("en" ""))
   "Alist of languages and format strings for the HTML preamble.
 
+To enable the HTML exporter to use these formats, you need to set
+`org-export-html-preamble' to `t'.
+
 The first element of each list is the language code, as used for
 the #+LANGUAGE keyword.
 
@@ -510,6 +513,9 @@ precedence over this variable."
 "))
   "Alist of languages and format strings for the HTML postamble.
 
+To enable the HTML exporter to use these formats, you need to set
+`org-export-html-postamble' to `t'.
+
 The first element of each list is the language code, as used for
 the #+LANGUAGE keyword.