Add migration information for the section-based auth config

Also: add missing information about the changed host_config.

doc/migrations/5.0.0_5.1.0.md

@@ -0,0 +1,11 @@
+## Configuration changes
+
+The configuration format has slightly changed and you might need to amend `mongooseim.toml`.
+
+### Section `auth`
+
+Each authentication method needs a TOML section, e.g. if you have the `rdbms` method enabled, you need to have the `[auth.rdbms]` section in the configuration file, even if it is empty. The `methods` option is not required anymore and especially if you are using only one method, you can remove it. See the [configuration](../configuration/auth.md) for details.
+
+### Section `host_config`
+
+The rules for overriding global options in the `host_config` section have been simplified. The `auth` section and the `s2s.address`, `s2s.domain_certfile` and `s2s.shared` options now completely override the corresponding general settings instead of being merged with them.

mkdocs.yml

@@ -181,6 +181,7 @@ nav:
       - '4.0.1 to 4.1.0': 'migrations/4.0.1_4.1.0.md'
       - '4.1.0 to 4.2.0': 'migrations/4.1.0_4.2.0.md'
       - '4.2.0 to 5.0.0': 'migrations/4.2.0_5.0.0.md'
+      - '5.0.0 to 5.1.0': 'migrations/5.0.0_5.1.0.md'
       - 'MAM MUC migration helper': 'migrations/jid-from-mam-muc-script.md'
   - 'Contributions to the Ecosystem': 'Contributions.md'
   - 'MongooseIM History': 'History.md'