docs: document the `[required]` struct attribute (#6956)

doc/docs.md

@@ -1139,6 +1139,21 @@ Array and map fields are allocated.
 
 It's also possible to define custom default values.
 
+### Required fields
+
+```v
+struct Foo {
+    n int [required]
+}
+```
+
+You can mark a struct field with the `[required]` attribute, to tell V that
+that field must be initialized when creating an instance of that struct.
+
+This example will not compile, since the field `n` isn't explicitly initialized:
+```v failcompile
+_ = Foo{}
+```
 
 <a id='short-struct-initialization-syntax' />
 

vlib/v/checker/checker.v

@@ -589,7 +589,7 @@ pub fn (mut c Checker) struct_init(mut struct_init ast.StructInit) table.Type {
 						}
 					}
 					if !found {
-						c.error('field `${type_sym.source_name}.$field.name` is required',
+						c.error('field `${type_sym.source_name}.$field.name` must be initialized',
 							struct_init.pos)
 					}
 				}

vlib/v/checker/tests/struct_required_field.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/struct_required_field.vv:12:6: error: field `Abc.f3` is required
+vlib/v/checker/tests/struct_required_field.vv:12:6: error: field `Abc.f3` must be initialized
    10 |         f3: 789
    11 |     }
    12 |     _ = Abc{