orm: improve the README and the tests. Add an error message for foreign key tags (#15670)

vlib/orm/README.md

@@ -17,6 +17,7 @@
 - `[sql: 'name']` sets a custom column name for the field
 - `[sql_type: 'SQL TYPE']` sets the sql type which is used in sql
 - `[default: 'sql defaults']` sets the default value or function when create a new table
+- `[fkey: 'parent_id']` sets foreign key for an field which holds an array
 
 ## Usage
 
@@ -27,7 +28,15 @@ struct Foo {
     created_at  time.Time   [sql_type: 'DATETIME']
     updated_at  string      [sql_type: 'DATETIME']
     deleted_at  time.Time
+    children    []Child     [fkey: 'parent_id']
 }
+
+struct Child {
+    id        int    [primary; sql: serial]
+    parent_id int
+    name      string
+}
+
 ```
 
 ### Create
@@ -54,6 +63,14 @@ var := Foo{
     created_at: time.now()
     updated_at: time.now().str()
     deleted_at: time.now()
+    children: [
+        Child{
+            name: 'abc'
+        },
+        Child{
+            name: 'def'
+        },
+    ]
 }
 
 sql db {
@@ -65,7 +82,7 @@ sql db {
 
 ```v ignore
 sql db {
-    update Foo set name = 'cde' where name == 'abc'
+    update Foo set name = 'cde', updated_at = time.now() where name == 'abc'
 }
 ```
 
@@ -84,7 +101,7 @@ result := sql db {
 ```
 ```v ignore
 result := sql db {
-    select from Foo where id > 1 limit 5
+    select from Foo where id > 1 && name != 'lasanha' limit 5
 }
 ```
 ```v ignore