Improve docs on else keyword

[tsandall]

The docs currently show this example for the else keyword:

# Service queries "authorize" and intreprets result ("allow" or "deny")
authorize = "allow" {
    input.user = "superuser"
}

# Check if caller is from outside network.
else = "deny" {
    input.path[0] = "admin"
    input.source_network = "external"
}

# Check if caller is from inside network.
else = "allow" {
    input.path[0] = "admin"
    input.source_network = "internal"
} # ... more rules

From this example, it's not obvious that else is applied to the rule above (i.e., authorize).

# Service queries "authorize" and intreprets result ("allow" or "deny")
authorize = "allow" {
    input.user = "superuser"
} else = "deny" { 
    # Check if caller is from outside network.
    input.path[0] = "admin"
    input.source_network = "external"
} else = "allow" {
    # Check if caller is from inside network.
    input.path[0] = "admin"
    input.source_network = "internal"
} # ... more rules

We should replace the example with the second snippet above.