Improve/complete/generate documentation

**ENHANCEMENT**

I've tried to document some of the fibers modules (`timer`, `sched`, 'fiber` and `channel`) using the [LDoc format](https://stevedonovan.github.io/ldoc/). This will allow for the auto-generation of documentation and for contextual help in code editors.

However:
* the LDoc generation hasn't been tested
* the documentation isn't complete, and
* I'm not sure that the approach I've chosen (types I've created, etc) is the best way

This documentation task will require a good knowledge of the codebase and would be good to be done by someone other than me alongside code review.

Review the existing LDoc annotations for the timer, sched, fiber, and channel modules, then try generating the documentation with LDoc. Check whether generation works, identify incomplete module documentation, and assess whether the chosen types and approach accurately reflect the codebase.

Written by the indexing model from the issue text.