Documenting all the command line arguments

[prithv1]

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.