Improve statement on permissionless systems and PoW
Improve statement on permissionless systems and PoW

Rather than saying Proof of Work is used to verify identity, as any
entity with a valid key pair can participate in the network, PoW mining
validates transactions and secures the network. This is required when
dealing with unknown identities.

Fix Issue # FAB-6134.

Change-Id: Idd2a944c2a2fc09e8280fc041322751c034e235b
Signed-off-by: Dileban Karunamoorthy <29d56979b3ef75c6dca8b7dd8223f65d9e6ca311@gmail.com>

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