Small doc improvement about HTML pre/postamble.

* org-html.el (org-export-html-preamble-format)
(org-export-html-postamble-format): Improve the docstring.

* org.texi (HTML preamble and postamble): Small doc improvement.

Thanks to Zachary Jones who asked for this.

doc/org.texi

@@ -9929,11 +9929,11 @@ creates two levels of headings and does the rest as items.
 The HTML exporter lets you define a preamble and a postamble.
 
 The default value for @code{org-export-html-preamble} is @code{t}, which
-means that the preamble is inserted depending on the relevant formatting
-string in @code{org-export-html-preamble-format}.
+means that the preamble is inserted depending on the relevant format string
+in @code{org-export-html-preamble-format}.
 
 Setting @code{org-export-html-preamble} to a string will override the default
-formatting string.  Setting it to a function, will insert the output of the
+format string.  Setting it to a function, will insert the output of the
 function, which must be a string; such a function takes no argument but you
 can check against the value of @code{opt-plist}, which contains the list of
 publishing properties for the current file.  Setting to @code{nil} will not
@@ -9945,7 +9945,7 @@ means that the HTML exporter will look for the value of
 @code{org-export-creator-info} and @code{org-export-time-stamp-file},
 @code{org-export-html-validation-link} and build the postamble from these
 values.  Setting @code{org-export-html-postamble} to @code{t} will insert the
-postamble from the relevant formatting string found in
+postamble from the relevant format string found in
 @code{org-export-html-postamble-format}.  Setting it to @code{nil} will not
 insert any postamble.
 

lisp/org-html.el

@@ -382,11 +382,17 @@ precedence over this variable."
   :group 'org-export-html
   :type '(choice (const :tag "No preamble" nil)
 		 (const :tag "Default preamble" t)
-		 (string :tag "Custom formatting string")
+		 (string :tag "Custom format string")
 		 (function :tag "Function (must return a string)")))
 
 (defcustom org-export-html-preamble-format '(("en" ""))
-  "The format for the HTML preamble.
+  "Alist of languages and format strings for the HTML preamble.
+
+The first element of each list is the language code, as used for
+the #+LANGUAGE keyword.
+
+The second element of each list is a format string to format the
+preamble itself.  This format string can contain these elements:
 
 %t stands for the title.
 %a stands for the author's name.
@@ -416,8 +422,8 @@ precedence over this variable."
   :group 'org-export-html
   :type '(choice (const :tag "No postamble" nil)
 		 (const :tag "Auto preamble" 'auto)
-		 (const :tag "Default formatting string" t)
-		 (string :tag "Custom formatting string")
+		 (const :tag "Default format string" t)
+		 (string :tag "Custom format string")
 		 (function :tag "Function (must return a string)")))
 
 (defcustom org-export-html-postamble-format
@@ -426,7 +432,13 @@ precedence over this variable."
 <p class=\"creator\">Generated by %c</p>
 <p class=\"xhtml-validation\">%v</p>
 "))
-  "The format for the HTML postamble.
+  "Alist of languages and format strings for the HTML postamble.
+
+The first element of each list is the language code, as used for
+the #+LANGUAGE keyword.
+
+The second element of each list is a format string to format the
+postamble itself.  This format string can contain these elements:
 
 %a stands for the author's name.
 %e stands for the author's email.