user-manual: Fix issue with previous revert

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
runqueue: Fix race against tasks sharing stamp files

Shared work directories work by assuming bitbake will not run
more than one task with a specific stamp name. Recent runqueue optimisations
accidentally broke this meaning there could be races. This fixes the code.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>