Don't call drop in tcpstream docs

This suggests that you must call it, which is normally not what you want to do.
auto merge of #18818 : steveklabnik/rust/tcp_doc, r=alexcrichton

This suggests that you must call it, which is normally not what you want to do.

See here: http://www.reddit.com/r/rust/comments/2lpw9k/rust_irc_example_looking_for_feedback/clxnxxc?context=4