Rollup merge of #150658 - CieriA:patch-5, r=jhpratt

Clarify panic conditions in `Iterator::last`

Follow-up PR to #150580.

Now `Iterator::last`'s docs specify that it might panic only if the iterator is infinite, rather than if it has more than `usize::MAX` elements.

# Motivation
This is because `Iterator::last`, unlike `count`, also works on iterators with more than `usize::MAX` elements, but could panic on infinite iterators (as in `Repeat`), as discussed in #150580.

r? `@jhpratt`

Author: jhpratt

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

@@ -238,8 +238,7 @@ pub trait Iterator {
     ///
     /// # Panics
     ///
-    /// This function might panic if the iterator has more than [`usize::MAX`]
-    /// elements.
+    /// This function might panic if the iterator is infinite.
     ///
     /// # Examples
     ///