docs(vpc-gw): add documentation for dhcp arguments (scaleway#1139)

Author: remyleone

docs/resources/vpc_public_gateway_dhcp.md

@@ -23,6 +23,19 @@ The following arguments are supported:
 
 - `zone` - (Defaults to [provider](../index.md#zone) `zone`) The [zone](../guides/regions_and_zones.md#zones) in which the public gateway DHCP config should be created.
 - `project_id` - (Defaults to [provider](../index.md#project_id) `project_id`) The ID of the project the public gateway DHCP config is associated with.
+- `subnet` - (Required) The subnet to associate with the public gateway DHCP config.
+- `address` - (Optional) The IP address of the public gateway DHCP config.
+- `pool_low` - (Optional) Low IP (included) of the dynamic address pool. Defaults to the second address of the subnet.
+- `pool_high` - (Optional) High IP (excluded) of the dynamic address pool. Defaults to the last address of the subnet.
+- `enable_dynamic` - (Optional) Whether to enable dynamic pooling of IPs. By turning the dynamic pool off, only pre-existing DHCP reservations will be handed out. Defaults to `true`.
+- `valid_lifetime` - (Optional) For how long, in seconds, will DHCP entries will be valid. Defaults to 1h (3600s).
+- `renew_timer` - (Optional) After how long, in seconds, a renew will be attempted. Must be 30s lower than `rebind_timer`. Defaults to 50m (3000s).
+- `rebind_timer` - (Optional) After how long, in seconds, a DHCP client will query for a new lease if previous renews fail. Must be 30s lower than `valid_lifetime`. Defaults to 51m (3060s).
+- `push_default_route` - (Optional) Whether the gateway should push a default route to DHCP clients or only hand out IPs. Defaults to `true`.
+- `push_dns_server` - (Optional) Whether the gateway should push custom DNS servers to clients. This allows for instance hostname -> IP resolution. Defaults to `true`.
+- `dns_server_override` - (Optional) Override the DNS server list pushed to DHCP clients, instead of the gateway itself
+- `dns_search` - (Optional) Additional DNS search paths
+- `dns_local_name` - (Optional) TLD given to hostnames in the Private Network. Allowed characters are `a-z0-9-.`. Defaults to the slugified Private Network name if created along a GatewayNetwork, or else to `priv`.
 
 ## Attributes Reference