docs: the SWMR follow loop stops for a writer that died, not only one that closed

The README example looped while swmr_writer_active(), which never ends when
the writer crashed or was killed: libhdf5 clears the SWMR-write flag only on
close (the mid-write fixture keeps it set for good). The loop now also stops
after a minute without growth, and the README, the swmr_writer_active docs
(with the same loop as a compiled no_run doctest) and the design say why.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5/src/reader.rs

@@ -616,10 +616,34 @@ impl File {
         self.swmr.retries()
     }
 
-    /// Whether a SWMR writer has the file open now: the superblock is read
-    /// again and its SWMR-write flag returned. libhdf5 clears the flag when
-    /// the writer closes the file, so a reader can stop following it then
-    /// (and one last [`Dataset::refresh`] sees the final extents).
+    /// Whether a SWMR writer has the file open now, as far as the file
+    /// says: the superblock is read again and its SWMR-write flag returned.
+    /// libhdf5 clears the flag when the writer closes the file, so a reader
+    /// can stop following it then (and one last [`Dataset::refresh`] sees
+    /// the final extents). A writer that crashed or was killed never clears
+    /// it, so the flag alone is not a stop condition: give the loop another
+    /// one, such as a time without growth.
+    ///
+    /// ```no_run
+    /// # fn main() -> Result<(), clawhdf5::Error> {
+    /// use std::time::{Duration, Instant};
+    ///
+    /// let file = clawhdf5::File::open_swmr("live.h5")?;
+    /// let mut ds = file.dataset("samples")?;
+    /// let (mut seen, mut last_growth) = (0, Instant::now());
+    /// while file.swmr_writer_active()? && last_growth.elapsed() < Duration::from_secs(60) {
+    ///     ds.refresh()?;
+    ///     let n = ds.shape()?[0];
+    ///     if n > seen {
+    ///         // read rows seen..n ...
+    ///         (seen, last_growth) = (n, Instant::now());
+    ///     }
+    ///     std::thread::sleep(Duration::from_millis(100));
+    /// }
+    /// ds.refresh()?; // the final extent
+    /// # Ok(())
+    /// # }
+    /// ```
     pub fn swmr_writer_active(&self) -> Result<bool, Error> {
         self.retry(|| {
             let sb = Superblock::parse_in(&self.data, 0)?;