docs: updated documentation for task syntax

[joerdav]

Thanks doing that @PeteXC! These are really coherant.

My only comment before merging is on the documentation around headings.

In your docs around task names and task sections you outline that a task section must be a H2 and a task name must be a H3.

In actual fact a Task section can be any heading level, as long as the tasks underneath a one level higher.

[PeteXC]

No problem.

The changes you asked for have been made. Please let me know if there's anything else!