Vault rekeying documentation clarification

[jhmartin]

https://www.vaultproject.io/guides/rekeying-and-rotating.html#rekeying-vault states "Rekeying the Vault requires a quorum of unseal keys. Before continuing, you should ensure all unseal key holders are available to assist with the rekeying." (emphasis mine)

The presence of the word 'all' is confusing, as in my test on 0.8.2 where I had initialized with shares=3 and threshold=2, I was able to rekey with just 2. This line should clarify if a majority is required or if just $threshold is required.

It should also clarify that rekeying does not affect the seal status of any other nodes in the cluster.