Rollup merge of #148609 - AMDmi3:rsplit_once_example, r=chenyukang

Sync str::rsplit_once example with str::split_once

This adds `"cfg=".rsplit_once('=')` case to `rsplit_once` example, bringing it in sync with example for `split_once`. For consistency and to make life easier for ones who want to ensure bahaviour of this specific edge case.

Author: jhpratt

library/core/src/str/mod.rs

@@ -1953,6 +1953,7 @@ impl str {
     ///
     /// ```
     /// assert_eq!("cfg".rsplit_once('='), None);
+    /// assert_eq!("cfg=".rsplit_once('='), Some(("cfg", "")));
     /// assert_eq!("cfg=foo".rsplit_once('='), Some(("cfg", "foo")));
     /// assert_eq!("cfg=foo=bar".rsplit_once('='), Some(("cfg=foo", "bar")));
     /// ```