Updated docs to reflect the task argument / environment variable matc…

…hup.

doc/rakefile.rdoc

@@ -183,6 +183,22 @@ argument string should be quoted.  Something like this:
 (Quoting rules vary between operating systems and shells, so make sure
 you consult the proper docs for your OS/shell).
 
+=== Tasks Arguments and the Environment
+
+Task argument values can also be picked up from the environment.  For
+example, if the "release" task expected a parameter named
+"release_version", then either
+
+   rake release[0.8.2]
+
+or
+
+   RELEASE_VERSION rake release
+
+will work.  Environment variable names must either match the task
+parameter exactly, or match an all uppcase version of the task
+parameter.
+
 === Tasks that Expect Parameters
 
 Parameters are only given to tasks that are setup to expect them.  In
@@ -210,7 +226,8 @@ The first argument of the block "t" is always bound to the current
 task object.  The second argument "args" is an open-struct like object
 that allows access to the task arguments.  Extra command line
 arguments to a task are ignored.  Missing command line arguments are
-given the nil value.
+picked up from matching environment variables.  If there are no
+matching environment variables, they are given the nil value.
 
 If you wish to specify default values for the arguments, you can use
 the with_defaults method in the task body.  Here is the above example
@@ -239,7 +256,8 @@ for tasks with arguments.  For example:
 There is an older format for declaring task parameters that omitted
 the task argument array and used the :needs keyword to introduce the
 dependencies.  That format is still supported for compatibility, but
-is not recommended for use.
+is not recommended for use.  The older format may be dropped in future
+versions of rake.
 
 == Accessing Task Programatically