docs: add implements keyword for explicit interface implementations (#22214)

doc/docs.md

@@ -3598,7 +3598,6 @@ fn main() {
 #### Implement an interface
 
 A type implements an interface by implementing its methods and fields.
-There is no explicit declaration of intent, no "implements" keyword.
 
 An interface can have a `mut:` section. Implementing types will need
 to have a `mut` receiver, for methods declared in the `mut:` section
@@ -3645,6 +3644,30 @@ fn fn1(s Foo) {
 // }
 ```
 
+There is an **optional** `implements` keyword for explicit declaration
+of intent, which applies to `struct` declarations.
+
+```v
+struct PathError implements IError {
+	Error
+	path string
+}
+
+fn (err PathError) msg() string {
+	return 'Failed to open path: ${err.path}'
+}
+
+fn try_open(path string) ! {
+	return PathError{
+		path: path
+	}
+}
+
+fn main() {
+	try_open('/tmp') or { panic(err) }
+}
+```
+
 #### Casting an interface
 
 We can test the underlying type of an interface using dynamic cast operators.