Update sync.md

[jimtalksdata]

Should be recover, not restore

[jimtalksdata]

Maybe should also include detailed conditions on how to manage:

• New nodes that just started w/ problems syncing
• Nodes that were in sync but did not enable sync_mode:normal(). previously

For future, all of this should be collapsed into sync: commands, and should run automatically for ease of use (maybe interactively ask the user whether he/she wants a light node or a full node)

[jimtalksdata]

There should also be a description of the difference between quick mode and normal mode sync

[zack-bitcoin]

Thank you