upgrade: Add Configuration Redesign section

Author: sirainen

docs/installation/upgrade/include/2.4-redesign.inc

@@ -14,3 +14,32 @@ to avoid unexpected storage file format incompatibilities.
 
 Note that the configuration syntax has been changed, and your old configuration **will not** work
 without changes.
+
+#### Configuration Redesign
+
+See [[link,settings_syntax]] for the new configuration syntax. This is similar
+to v2.3, but different in some ways. Especially the configuration is no longer
+hierarchical - all settings are global settings and can be used anywhere
+(although they might not actually do anything there). Settings can be inside
+various filters to specify where they are wanted to be used.
+
+To avoid repetition in setting name prefixes, they are automatically attempted
+to be prefixed to their parent filter names. For example these are equivalent
+for the [[setting,passdb_sql_query]] setting inside the [[setting,passdb]]
+filter:
+
+```
+passdb sql {
+  passdb_sql_query = SELECT ...
+}
+passdb sql {
+  sql_query = SELECT ...
+}
+passdb sql {
+  query = SELECT ...
+}
+passdb sql2 {
+  # This will NOT work, as it expands to nonexistent passdb_sql2_query:
+  #query = SELECT ...
+}
+```