toml: update value_opt syntax and add missing documentation (#16510)

vlib/toml/ast/ast.v

@@ -16,6 +16,7 @@ pub mut:
 	// errors           []errors.Error    // all the checker errors in the file
 }
 
+// str returns the string representation of the root node.
 pub fn (r Root) str() string {
 	mut s := typeof(r).name + '{\n'
 	s += '  input:  ${r.input}\n'

vlib/toml/ast/types.v

@@ -10,6 +10,8 @@ import strconv
 // can be found in a TOML document.
 pub type Key = Bare | Bool | Null | Number | Quoted
 
+// str returns the string representation of the key. This is implemented
+// by all the variants of Key.
 pub fn (k Key) str() string {
 	return k.text
 }

vlib/toml/ast/walker/walker.v

@@ -20,6 +20,7 @@ mut:
 	data voidptr
 }
 
+// visit calls the inspector callback on the specified Value node.
 pub fn (i &Inspector) visit(value &ast.Value) ! {
 	i.inspector_callback(value, i.data) or { return err }
 }