std::sync::TaskPool: Improve module documentation

The struct and module doc comments are reformulated. The `execute`
method's documentation are put up to date, and failure information
is added. A test is also added to address the possible failure.
auto merge of #14993 : alxgnon/rust/taskpooldocfix, r=alexcrichton

TaskPool is the only item in the std::sync doc that doesn't have a summary. This fixes that problem.