Check usage messages and reorder for best docs: remaining files

[adrianVmariano]

These files need to be checked for usage text correctness according to our latest conventions, so with indication of positional vs named args, with [ATTACHMENTS]; if appropriate, possibly check for no_children($children) if not appropriate and make a no_function() version if appropriate.

• [x] joiners.scad
• [ ] sliders.scad
• [ ] linear_bearings.scad
• [ ] cubetruss.scad
• [ ] gears.scad
• [x] nema_steppers.scad
• [ ] fnliterals.scad
• [ ] metric_screws.scad

Make sure also that modules and functions are presented in a natural order for the docs. In cubetruss should the main truss be first? Also I think cubetruss may need a better example for the clips. I can't tell WTF they actually do out of context.

Also make sure all exposed functions should be exposed, especially in gears.scad and nema_steppers.scad.