Add `.write(true)` to append and truncate examples

Setting append without write doesn't give you a writeable file. Showing
it as an example in the docs is confusing at best.

Using truncate on a read-only file is an error on POSIX systems (note
however that using create with read-only flags is fine).

src/libstd/fs.rs

@@ -419,7 +419,7 @@ impl OpenOptions {
     /// ```no_run
     /// use std::fs::OpenOptions;
     ///
-    /// let file = OpenOptions::new().append(true).open("foo.txt");
+    /// let file = OpenOptions::new().write(true).append(true).open("foo.txt");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn append(&mut self, append: bool) -> &mut OpenOptions {
@@ -436,7 +436,7 @@ impl OpenOptions {
     /// ```no_run
     /// use std::fs::OpenOptions;
     ///
-    /// let file = OpenOptions::new().truncate(true).open("foo.txt");
+    /// let file = OpenOptions::new().write(true).truncate(true).open("foo.txt");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn truncate(&mut self, truncate: bool) -> &mut OpenOptions {