Improve docs for std::io::Seek

Fixes #54562

src/libstd/io/mod.rs

@@ -1204,8 +1204,8 @@ pub trait Write {
 pub trait Seek {
     /// Seek to an offset, in bytes, in a stream.
     ///
-    /// A seek beyond the end of a stream is allowed, but implementation
-    /// defined.
+    /// A seek beyond the end of a stream is allowed, but behavior is defined
+    /// by the implementation.
     ///
     /// If the seek operation completed successfully,
     /// this method returns the new position from the start of the stream.