Update RDoc::Markup to explain rdoc-ref: and fix unclosed tt

Author: drbrain

lib/rdoc/markup.rb

@@ -284,7 +284,19 @@
 #
 # Links can also be of the form <tt>label[url]</tt>, in which case +label+ is
 # used in the displayed text, and +url+ is used as the target.  If +label+
-# contains multiple words, put it in braces: <tt>{multi word label}[url]<tt>.
+# contains multiple words, put it in braces: <tt>{multi word label}[url]</tt>.
+# The +url+ may be an +http:+-type link or a cross-reference to a class,
+# module or method with a label.
+#
+# Links with the <tt>rdoc-ref:</tt> scheme will link to the referenced class,
+# module, method, file, etc.  If the referenced item is does not exist
+# no link will be generated and <tt>rdoc-ref:</tt> will be removed from the
+# resulting text.
+#
+# Links starting with +link:+ refer to local files whose path is relative to
+# the <tt>--op</tt> directory.  Use <tt>rdoc-ref:</tt> instead of
+# <tt>link:</tt> to link to files generated by RDoc as the link target may
+# be different across RDoc generators.
 #
 # Example links:
 #