This change updates the \fn commands in the QtConcurrent
module, which makes extensive use of templates. Some of the
update \fn commands are very long as a result.

A few cases of the tag trick were used to fix qdoc errors
for some complicated template function signatures that were
marked \internal anyway.

Change-Id: I8546b89030c51bb9dddf38fa10bc0248d901aa51
Reviewed-by: Martin Smith <martin.smith@qt.io>