Add a further explaination on ltr and rtl (based on ePub).

Suggested by w3c/rdf-turtle#75.

Author: gkellogg

spec/index.html

@@ -759,6 +759,12 @@ <h2>Literals</h2>
       The third element, the language tag, is treated identically as in a <a>language-tagged string</a>,
       and the fourth element, <a>base direction</a>, MUST be either `ltr` or `rtl`, which MUST be in lower case.</p>
 
+    <p>The meanings of the <a>base direction</a> values are:</p>
+    <ul>
+      <li>`ltr`: indicates that the textual value is explicitly directionally set to left-to-right text.</li>
+      <li>`rtl`: indicates that the textual value is explicitly directionally set to right-to-left text.</li>
+    </ul>
+
     <p>Please note that concrete syntaxes MAY support
       <dfn data-lt="simple literal" class="export">simple literals</dfn> consisting of only a
       <a>lexical form</a> without any <a>datatype IRI</a>, <a>language tag</a>, or <a>base direction</a>.