Update usage note on OpenOptions::append()

Avoid implying that concatenating data before passing it to `write()` (with
or without `BufWriter`) ensures atomicity.

library/std/src/fs.rs

@@ -980,15 +980,21 @@ pub fn write(&mut self, write: bool) -> &mut Self {
     /// Note that setting `.write(true).append(true)` has the same effect as
     /// setting only `.append(true)`.
     ///
-    /// For most filesystems, the operating system guarantees that all writes are
-    /// atomic: no writes get mangled because another process writes at the same
-    /// time.
+    /// Append mode guarantees that writes will be positioned at the current end of file,
+    /// even when there are other processes or threads appending to the same file. This is
+    /// unlike <code>[seek]\([SeekFrom]::[End]\(0))</code> followed by `write()`, which
+    /// has a race between seeking and writing during which another writer can write, with
+    /// our `write()` overwriting their data.
     ///
-    /// One maybe obvious note when using append-mode: make sure that all data
-    /// that belongs together is written to the file in one operation. This
-    /// can be done by concatenating strings before passing them to [`write()`],
-    /// or using a buffered writer (with a buffer of adequate size),
-    /// and calling [`flush()`] when the message is complete.
+    /// Keep in mind that this does not necessarily guarantee that data appended by
+    /// different processes or threads does not interleave. The amount of data accepted a
+    /// single `write()` call depends on the operating system and file system. A
+    /// successful `write()` is allowed to write only part of the given data, so even if
+    /// you're careful to provide the whole message in a single call to `write()`, there
+    /// is no guarantee that it will be written out in full. If you rely on the filesystem
+    /// accepting the message in a single write, make sure that all data that belongs
+    /// together is written in one operation. This can be done by concatenating strings
+    /// before passing them to [`write()`].
     ///
     /// If a file is opened with both read and append access, beware that after
     /// opening, and after every write, the position for reading may be set at the
@@ -1003,6 +1009,9 @@ pub fn write(&mut self, write: bool) -> &mut Self {
     /// [`write()`]: Write::write "io::Write::write"
     /// [`flush()`]: Write::flush "io::Write::flush"
     /// [stream_position]: Seek::stream_position "io::Seek::stream_position"
+    /// [seek]: Seek::seek "io::Seek::seek"
+    /// [Current]: SeekFrom::Current "io::SeekFrom::Current"
+    /// [End]: SeekFrom::End "io::SeekFrom::End"
     ///
     /// # Examples
     ///