document `arg` var for commandline arguments

doc/manual.html

@@ -2181,7 +2181,7 @@ each library provides all its functions as fields of a global table
 or as methods of its objects.
 
 
-<h2>5.1 - <a name="5.1">Basic Functions</a></h2>
+<h2>5.1 - <a name="5.1">Basic Primitives</a></h2>
 
 <p>
 The basic library provides some core functions to Lua.
@@ -2190,6 +2190,11 @@ you should check carefully whether you need to provide
 implementations for some of its facilities.
 
 
+<p>
+<hr><h3><a name="pdf-arg"><code>arg</code></a></h3>
+A global variable containing any commandline arguments.
+
+
 <p>
 <hr><h3><a name="pdf-assert"><code>assert (v [, message])</code></a></h3>
 Issues an  error when
@@ -2277,7 +2282,7 @@ to the message.
 
 <p>
 <hr><h3><a name="pdf-_G"><code>_G</code></a></h3>
-A global variable (not a function) that
+A global variable that
 holds the global environment (that is, <code>_G._G = _G</code>).
 Lua itself does not use this variable;
 changing its value does not affect any environment,