Write docs; Add missing docs warning

docs/language.md

@@ -353,6 +353,19 @@ stdout_message = new Message {
 The `option(type)` type is expected to be either `some(value)` or `none`. The type of the value
 inside the `some` is always specified.
 
+```dust
+result <option(str)> = none
+
+for file in fs:read_dir("./") {
+    if file:size > 100 {
+        result = some(file:path)
+        break
+    }
+}
+
+output(result)
+```
+
 ```dust
 get_line_break_index(text <str>) <some(int)> {
     str:find(text, '\n')
@@ -364,7 +377,7 @@ get_line_break_index(text <str>) <some(int)> {
 Custom types such as **structures** are referenced by their variable identifier.
 
 ```dust
-struct File {
+File = struct {
     path <str>
     size <int>
     type <str>

src/lib.rs

@@ -1,3 +1,5 @@
+#![warn(missing_docs)]
+
 //! The Dust library is used to parse, format and run dust source code.
 //!
 //! See the [interpret] module for more information.