docs: Reword slice::strip_prefix and strip_suffix a bit

The stabilisation issue, rust-lang#73413, has an open item for documentation.
I looked at the docs and it is all there, but I felt it could do with
some minor wording improvement.

I looked at the `str::strip_prefix` docs for a template.  (That
resulted in me slightly changing that doc too.)

I de-linkified `None` and `Some`, as I felt that rather noisy..  I
searched stdlib, and these don't seem to be usually linkified.

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

Author: ijackson

library/core/src/slice/mod.rs

@@ -1703,8 +1703,12 @@ impl<T> [T] {
 
     /// Returns a subslice with the prefix removed.
     ///
-    /// This method returns [`None`] if slice does not start with `prefix`.
-    /// Also it returns the original slice if `prefix` is an empty slice.
+    /// If the slice starts with `prefix`, returns
+    /// the subslice after the prefix, wrapped in `Some`.
+    ///
+    /// If the slice does not start with `prefix`, returns `None`.
+    ///
+    /// (If `prefix` is empty, simply returns the original slice.)
     ///
     /// # Examples
     ///
@@ -1734,8 +1738,12 @@ impl<T> [T] {
 
     /// Returns a subslice with the suffix removed.
     ///
-    /// This method returns [`None`] if slice does not end with `suffix`.
-    /// Also it returns the original slice if `suffix` is an empty slice
+    /// If the slice ends with `suffix`, returns
+    /// the subslice before the suffix, wrapped in `Some`.
+    ///
+    /// If the slice does not end with `suffix`, returns `None`.
+    ///
+    /// (If `suffix` is empty, simply returns the original slice.)
     ///
     /// # Examples
     ///