rustdoc: Fix rendering closures and trait bounds

Closures did not have their bounds printed at all, nor their lifetimes. Trait
bounds were also printed in angle brackets rather than after a colon with a '+'
inbetween them.

Note that on the current task::spawn [1] documentation page, there is no mention
of a `Send` bound even though it is crucially important!

[1] - http://static.rust-lang.org/doc/master/std/task/fn.task.html
auto merge of #13464 : alexcrichton/rust/fix-rustdoc-rendering, r=brson

Closures did not have their bounds printed at all, nor their lifetimes. Trait
bounds were also printed in angle brackets rather than after a colon with a '+'
inbetween them.

Note that on the current task::spawn [1] documentation page, there is no mention
of a `Send` bound even though it is crucially important!

[1] - http://static.rust-lang.org/doc/master/std/task/fn.task.html