Document an edge case of str::split_once

Author: imbolc

library/core/src/str/mod.rs

@@ -1632,6 +1632,7 @@ impl str {
     ///
     /// ```
     /// assert_eq!("cfg".split_once('='), None);
+    /// assert_eq!("cfg=".split_once('='), Some(("cfg", "")));
     /// assert_eq!("cfg=foo".split_once('='), Some(("cfg", "foo")));
     /// assert_eq!("cfg=foo=bar".split_once('='), Some(("cfg", "foo=bar")));
     /// ```