Update tyk-sync documentation

Hey I just noticed that there are options available in the tyk-sync tool which are not documented in the official documentation at https://tyk.io/docs/tyk-sync. Specifically the --apis and --policies options are only documented for the dump command but appear to also be available for all other commands.

Additionally under the Tyk Sync section the update command seems to be using a default string from the arg parser rather than actually including a description of what it does.

Thanks for bringing this to our attention @mpb01e

We will make the necessary changes and update the documentation accordingly.