Give the asio documentation its own target.

The asio documentation is built with the rest of the combined documentation but
is really separate. So give it its own target so that separate parts can be
built separately.


[SVN r46742]

doc/Jamfile.v2

@@ -6,6 +6,8 @@
 project boost/doc ;
 import boostbook : boostbook ;
 
+alias asio : ../libs/asio/doc//asio ;
+
 boostbook doc
     :
     src/boost.xml
@@ -41,7 +43,6 @@ boostbook doc
     <dependency>../libs/intrusive/doc//intrusive
     <dependency>../libs/units/doc//units
     <dependency>../libs/unordered/doc//unordered
-    <dependency>../libs/asio/doc//asio
     <dependency>../libs/thread/doc//thread
 
     ## Add path references to the QuickBook generated docs...