Update sync SIP

[y0sher]

sync doc seems to be outdated after we introduced remove decided requirement. We should either update this to reflect the new sync state, or write a new doc about the current state of sync (only full nodes sync).

@GalRogozinski @MatheusFranco99 @alonmuroch

[GalRogozinski]

so in short...
the protocol doesn't need syncing

If the implementation wants syncing (because it wants to be a full node) it can implement such features.

Since a full node may not be able to sync from a node that doesn't support such features I would recommend peering several full nodes together.

@olegshmuelov

[GalRogozinski]

The sync doc should be deprecated