Document the current behaviour of clear_last_lines when called with a…

… too large argument.

src/term.rs

@@ -449,6 +449,10 @@ impl Term {
     /// Clear the last `n` lines before the current line.
     ///
     /// Position the cursor at the beginning of the first line that was cleared.
+    ///
+    /// **Caution.** When `n` is larger than the number of lines above the
+    /// current cursor, the top `n` lines are cleared --- including some lines
+    /// below the cursor.
     pub fn clear_last_lines(&self, n: usize) -> io::Result<()> {
         self.move_cursor_up(n)?;
         for _ in 0..n {