Give a better explanation of the lockhash in terminology.rst

[loredanacirstea]

Explain lockhash better in terminology.rst, or give at least an example of what the lock is or contains.
Now:

The hash of a lock. sha3_keccack(lock)

Code reference: https://github.com/raiden-network/raiden/blob/5dc039ccc0c51d02534863ef0bd40dbbf9954627/raiden/transfer/state.py#L671