library: Document panic condition for Iterator::last

Added a `# Panics` section to `Iterator::last` to match `Iterator::count`, after the change made in Rust 1.92.0 where now `std::iter::Repeat` panics in these methods instead of doing an infinite `loop`.

Cites https://github.com/rust-lang/rust/issues/149707

library/core/src/iter/traits/iterator.rs

@@ -236,6 +236,11 @@ pub trait Iterator {
     /// doing so, it keeps track of the current element. After [`None`] is
     /// returned, `last()` will then return the last element it saw.
     ///
+    /// # Panics
+    ///
+    /// This function might panic if the iterator has more than [`usize::MAX`]
+    /// elements.
+    ///
     /// # Examples
     ///
     /// ```