Add note about append not implying write

Hopefully this will reduce the kind of confusion @joelmccracken repots in
http://joelmccracken.github.io/entries/a-simple-web-app-in-rust-pt-2b/

Author: steveklabnik

src/libstd/fs.rs

@@ -422,6 +422,11 @@ impl OpenOptions {
     /// This option, when true, means that writes will append to a file instead
     /// of overwriting previous contents.
     ///
+    /// Note that this isn't the same as `write` with the cursor starting at
+    /// the end of the file; this mode only allows writing to the end of the
+    /// file, but not changing any of its previous contents. This is common for
+    /// things like log files.
+    ///
     /// # Examples
     ///
     /// ```no_run