Documenting all the command line arguments

## Problem

[Creating an issue here following Luca's suggestion]
It might be beneficial for users if all the command line arguments in the `main.py` script were documented in the AllenAct docs. I ran into an issue (fixed now) where I accidentally started training from the latest checkpoint with the `restart_pipeline` flag for a run that had crashed prematurely -- my goal was to resume training as opposed starting an entirely new training run from a pre-trained checkpoint. Clarifying when to use different command line arguments (beyond the ones which are already documented) will be helpful.

## Desired solution

Explicitly documenting all the command line arguments.