[DOC] Improve documentation of include directive file search

Searching the file in the "current directory" could be read as the
directory from which RDoc is run. This is incorrect, as the file is
first searched in the directory containing the current documented file.

Also this directory is searched before the `--include` directories.

doc/rdoc/markup_reference.rb

@@ -641,9 +641,9 @@
 #     The file content is shifted to have the same indentation as the colon
 #     at the start of the directive.
 #
-#     The file is searched for in the directories
-#     given with the <tt>--include</tt> command-line option,
-#     or by default in the current directory.
+#     The file is searched for in the directory containing the current file,
+#     and then in each of the directories given with the <tt>--include</tt>
+#     command-line option.
 #
 #   For C code, the directive may appear in a stand-alone comment
 #