IPv6 link-local address with scope

[Wang Shanker]

Hi, all

I encountered a problem when trying to configure a static IPv6 route the nexthop of which is a link-local address. 

```
protocol static {
  route 2001:db8:1::/48 via fe80::1234%foo;
}
```

The above configuration worked well for routing traffic to 2001:db8:1::/48 to the router fe80::1234 at the interface named foo. However, if the name of the interface contains special characters, for example, `foo-bar`, bird will complain about the syntax. 

I dig into the source code and found out that in https://gitlab.nic.cz/labs/bird/-/blob/legacy/conf/confbase.Y#L162-165, `ipa_scope` is defined like `%[SYM]`, so in above scenario, the address shall be expressed as `fe80::1234%'foo-bar'`. 

I believe it could be better to have a record of this in user's doc. 

Cheers,

Miao Wang