org-manual: Rename "Markup for Rich Export"

* doc/org-manual.org (Markup for Rich Contents): Renamed from "Markup
  for Rich Export".  Rewrite the introduction and the description.
(Exporting): Fix cross-reference.

At this point, the export framework hasn't been described so it could
be confusing to use it in the section name.

doc/org-manual.org

@@ -10534,17 +10534,19 @@ environment.  This causes the following issues:
    today, with the time already spent---via =CLOCKSUM=---and with
    the planned total effort for it.
 
-* Markup for Rich Export
+* Markup for Rich Contents
 :PROPERTIES:
-:DESCRIPTION: Prepare text for rich export.
-:ALT_TITLE: Markup
+:DESCRIPTION: Compose beautiful documents.
 :END:
 
-When exporting Org documents, the exporter tries to reflect the
-structure of the document as accurately as possible in the back-end.
-Since export targets like HTML and LaTeX allow much richer formatting,
-Org mode has rules on how to prepare text for rich export.  This
-section summarizes the markup rules used in an Org mode buffer.
+As you could tell so far, Org is primarily about organizing and
+searching through your notes.  However, it also provides a lightweight
+markup language that allows a rich formatting for those.  You may want
+to center, or emphasize, text.  You may need to insert a formula or
+a picture within your writings.  There is some syntax for all of this.
+Used in conjuction with the export framework (see [[*Exporting]]), you can
+author appealing documents, like the fine manual you are currently
+reading!
 
 ** Paragraphs
 :PROPERTIES:
@@ -11098,7 +11100,7 @@ Sometimes, you may want to pretty print your notes, publish them on
 the web or even share them with people not using Org.  In these cases,
 the Org export facilities can be used to convert your documents to
 a variety of other formats, while retaining as much structure (see
-[[*Document Structure]]) and markup (see [[*Markup for Rich Export]]) as
+[[*Document Structure]]) and markup (see [[*Markup for Rich Contents]]) as
 possible.
 
 #+cindex: export back-end