docs: slice::strip_prefix and strip_suffix, fold in sentence

Roughly as requested by @LukasKalbertodt.  I still prefer clearly
making these two cases.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/core/src/slice/mod.rs

@@ -1704,11 +1704,10 @@ impl<T> [T] {
     /// Returns a subslice with the prefix removed.
     ///
     /// If the slice starts with `prefix`, returns the subslice after the prefix, wrapped in `Some`.
+    /// If `prefix` is empty, simply returns the original slice.
     ///
     /// If the slice does not start with `prefix`, returns `None`.
     ///
-    /// (If `prefix` is empty, simply returns the original slice.)
-    ///
     /// # Examples
     ///
     /// ```
@@ -1738,11 +1737,10 @@ impl<T> [T] {
     /// Returns a subslice with the suffix removed.
     ///
     /// If the slice ends with `suffix`, returns the subslice before the suffix, wrapped in `Some`.
+    /// If `suffix` is empty, simply returns the original slice.
     ///
     /// If the slice does not end with `suffix`, returns `None`.
     ///
-    /// (If `suffix` is empty, simply returns the original slice.)
-    ///
     /// # Examples
     ///
     /// ```