Weaken guarantee for From<legacy::RangeInclusive> for RangeInclusive

As per https://github.com/rust-lang/rust/pull/155114#issuecomment-4845403701,
this `From` impl no longer guarantees panicking for exhausted iterators.
Instead, it only guarantees that the conversion will either panic or
produce an empty range.

This is done so that we can optimize the implementation of
`legacy::RangeInclusive` in a way such that we cannot check if it has
been exhausted in a generic context without a `Step` and/or `PartialOrd`
bound.

library/core/src/range.rs

@@ -402,7 +402,8 @@ const impl<T> From<legacy::RangeInclusive<T>> for RangeInclusive<T> {
     ///
     /// # Panics
     ///
-    /// Panics if the legacy range iterator has been exhausted.
+    /// If the legacy range iterator has been exhausted,
+    /// this function will either panic or return an empty range.
     ///
     /// # Examples
     ///
@@ -419,19 +420,16 @@ const impl<T> From<legacy::RangeInclusive<T>> for RangeInclusive<T> {
     /// assert_eq!((empty.start, empty.last), (0, 0));
     /// ```
     ///
-    /// ```should_panic
+    /// ```
     /// use core::range::legacy;
     /// use core::range::RangeInclusive;
+    /// use std::panic::catch_unwind;
     ///
     /// let mut exhausted: legacy::RangeInclusive<i32> = 0..=0;
     /// exhausted.next();
-    /// # if exhausted.is_empty() {
-    /// # // assert!s don't work correctly in `should_panic` doctests since you
-    /// # // can't assert the panic message. Skip the rest of the test instead,
-    /// # // so that the expected panic doesn't happen and the test fails.
-    /// assert!(exhausted.is_empty());
-    /// let _ = RangeInclusive::from(exhausted); // this panics
-    /// # }
+    /// let result = catch_unwind(|| RangeInclusive::from(exhausted));
+    /// // The `from` call either panicked or returned an empty range.
+    /// assert!(result.is_err() || result.is_ok_and(|range| range.is_empty()));
     /// ```
     #[inline]
     fn from(value: legacy::RangeInclusive<T>) -> Self {