Document the alternate format for :ref:.

Doc/documenting/markup.rst

@@ -610,6 +610,8 @@ Example::
 
 The ``:ref:`` invocation is replaced with the section title.
 
+Alternatively, you can reference any label (not just section titles)
+if you provide the link text ``:ref:`link text`<reference-label>```.
 
 Paragraph-level markup
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