[Docs] Logging section is incomplete

[valenting]

The logging section of the 0.6 docs ( http://static.rust-lang.org/doc/0.6/rust.html#logging-system ) doesn't specify how you log messages.
This link is broken:
http://static.rust-lang.org/doc/0.6/rust.html#log-expressions
It is not broken in the 0.5 docs, but I believe the API has changed.
http://static.rust-lang.org/doc/0.5/rust.html#log-expressions