network: clarify the relationship between RA flags and DHCPv6 modes

In the documentation, using the term "managed" for both the RA flag and
the DHCPv6 mode is confusing because the mode is referred to as
"solicit" both in the official DHCPv6 documentation (see RFC 8415) and
in the WithoutRA option.

Furthermore, calling the other RA flag "other information" or "other
address configuration" is confusing because its official name is simply
"other configuration" (see RFC 4861 and RFC 5175) and it isn't used to
assign IP addresses.

Rewrite the documentation for DHCPv6Client and WithoutRA to make it
clear that getting the "managed" RA flag triggers the same kind of DHCP
request as WithoutRA=solicit, whereas getting the "other configuration"
RA flag triggers the same kind of DHCP request as
WithoutRA=information-request.

src/libsystemd-network/sd-dhcp6-client.c

@@ -1384,7 +1384,7 @@ int sd_dhcp6_client_start(sd_dhcp6_client *client) {
         }
 
         log_dhcp6_client(client, "Starting in %s mode",
-                         client->information_request ? "Information request" : "Managed");
+                         client->information_request ? "Information request" : "Solicit");
 
         return client_start_transaction(client, state);
 }