docs: add more documentation to each of the modules in vlib (#13043)

vlib/clipboard/README.md

@@ -3,3 +3,14 @@
 `clipboard` provides access to the platform's clipboard mechanism.
 You can use it to read from the system clipboard, and write to it
 from your applications.
+
+## Examples:
+
+```v
+import clipboard
+
+fn main() {
+	mut c := clipboard.new()
+	println(c.get_text())
+}
+```

vlib/clipboard/clipboard.v

@@ -21,7 +21,7 @@ pub fn (mut cb Clipboard) clear_all() {
 	cb.clear()
 }
 
-// destroy destroys the clipboard and free it's resources.
+// destroy destroys the clipboard and frees its resources.
 pub fn (mut cb Clipboard) destroy() {
 	cb.free()
 }