Added a boostdoc target to unify Boost release docs building.

doc/Jamfile.v2

@@ -51,3 +51,6 @@ boostbook standalone
     <xsl:param>toc.max.depth=1
     <format>pdf:<xsl:param>boost.url.prefix=http://www.boost.org/doc/libs/release/libs/core/doc/html
   ;
+
+alias boostdoc : standalone
+    : : : <implicit-dependency>standalone ;