Clean up std::task docs, make TaskBuilder a real builder

Delete all the documentation from std::task that references linked
failure.

Tweak TaskBuilder to be more builder-like. .name() is now .named() and
.add_wrapper() is now .with_wrapper(). Remove .watched() and
.unwatched() as they didn't actually do anything.
Update clients of the TaskBuilder API
auto merge of #12232 : kballard/rust/taskbuilder-is-a-builder, r=alexcrichton

Delete all the documentation from std::task that references linked
failure.

Tweak TaskBuilder to be more builder-like. `.name()` is now `.named()` and
`.add_wrapper()` is now `.with_wrapper()`. Remove `.watched()` and
`.unwatched()` as they didn't actually do anything.

Closes #6399.