Multiple errors/inconsistencies in curtin network documentation

[klausfiend]

The documentation for the "network" section of curtin has several discrepancies with the actual implementation, specifically:

- the keyword `xmit_hash_policy` should actually be `transmit_hash_policy`
- the documentation references `mode` and `bond-mode` interchangeably (although there is no mention of `bond_mode` but presumably it can be used identically to `bond-mode`)

I'm assuming some of this disagreement stems from the kernel documentation for bonding itself, which references `xmit_hash_policy`, but this configuration does not work:

```
# This is the network config written by 'subiquity'
network:
  bonds:
    bond0:
      addresses:
      - <redacted>
      gateway4: <redacted>
      interfaces:
      - eth0
      - eth1
      macaddress: <redacted>
      nameservers:
        addresses:
        - <redacted>
        - <redacted>
        search:
        - <redacted>
      parameters:
        xmit-hash-policy: layer3+4
        lacp-rate: fast
        mii-monitor-interval: 100
        mode: 802.3ad
  ethernets:
    eth0: {}
    eth1: {}
  version: 2
```

whereas this nearly identical configuration works as expected:

```
# This is the network config written by 'subiquity'
network:
  bonds:
    bond0:
      addresses:
      - <redacted>
      gateway4: <redacted>
      interfaces:
      - eth0
      - eth1
      macaddress: <redacted>
      nameservers:
        addresses:
        - <redacted>
        - <redacted>
        search:
        - <redacted>
      parameters:
        transmit-hash-policy: layer3+4
        lacp-rate: fast
        mii-monitor-interval: 100
        mode: 802.3ad
  ethernets:
    eth0: {}
    eth1: {}
  version: 2
```

The attached patch normalizes parameter keyword usage and corrects the hash policy keyword to agree with the underlying implementation.