io: clarify behavior of seeking when start_seek is not used (#7366)

tokio/src/io/async_seek.rs

@@ -34,11 +34,17 @@ pub trait AsyncSeek {
 
     /// Waits for a seek operation to complete.
     ///
-    /// If the seek operation completed successfully,
+    /// If the seek operation completed successfully, this method returns the
-    /// this method returns the new position from the start of the stream.
+    /// new position from the start of the stream. That position can be used
-    /// That position can be used later with [`SeekFrom::Start`]. Repeatedly
+    /// later with [`SeekFrom::Start`].
-    /// calling this function without calling `start_seek` might return the
+    ///
-    /// same result.
+    /// The position returned by calling this method can only be relied on right
+    /// after `start_seek`. If you have changed the position by e.g. reading or
+    /// writing since calling `start_seek`, then it is unspecified whether the
+    /// returned position takes that position change into account. Similarly, if
+    /// `start_seek` has never been called, then it is unspecified whether
+    /// `poll_complete` returns the actual position or some other placeholder
+    /// value (such as 0).
     ///
     /// # Errors
     ///