Clarify behavior of slice prefix/suffix operations in case of equality

Operations such as starts_with, ends_with, strip_prefix and strip_suffix
can be either strict (do not consider a slice to be a prefix/suffix of
itself) or not. In Rust's case, they are not strict. Add a few phrases to
the documentation to clarify this.

library/core/src/slice/mod.rs

@@ -2519,7 +2519,7 @@ impl<T> [T] {
         cmp::SliceContains::slice_contains(x, self)
     }
 
-    /// Returns `true` if `needle` is a prefix of the slice.
+    /// Returns `true` if `needle` is a prefix of the slice or equal to the slice.
     ///
     /// # Examples
     ///
@@ -2527,6 +2527,7 @@ impl<T> [T] {
     /// let v = [10, 40, 30];
     /// assert!(v.starts_with(&[10]));
     /// assert!(v.starts_with(&[10, 40]));
+    /// assert!(v.starts_with(&v));
     /// assert!(!v.starts_with(&[50]));
     /// assert!(!v.starts_with(&[10, 50]));
     /// ```
@@ -2549,7 +2550,7 @@ impl<T> [T] {
         self.len() >= n && needle == &self[..n]
     }
 
-    /// Returns `true` if `needle` is a suffix of the slice.
+    /// Returns `true` if `needle` is a suffix of the slice or equal to the slice.
     ///
     /// # Examples
     ///
@@ -2557,6 +2558,7 @@ impl<T> [T] {
     /// let v = [10, 40, 30];
     /// assert!(v.ends_with(&[30]));
     /// assert!(v.ends_with(&[40, 30]));
+    /// assert!(v.ends_with(&v));
     /// assert!(!v.ends_with(&[50]));
     /// assert!(!v.ends_with(&[50, 30]));
     /// ```
@@ -2582,7 +2584,8 @@ 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 `prefix` is empty, simply returns the original slice. If `prefix` is equal to the
+    /// original slice, returns an empty slice.
     ///
     /// If the slice does not start with `prefix`, returns `None`.
     ///
@@ -2592,6 +2595,7 @@ impl<T> [T] {
     /// let v = &[10, 40, 30];
     /// assert_eq!(v.strip_prefix(&[10]), Some(&[40, 30][..]));
     /// assert_eq!(v.strip_prefix(&[10, 40]), Some(&[30][..]));
+    /// assert_eq!(v.strip_prefix(&[10, 40, 30]), Some(&[][..]));
     /// assert_eq!(v.strip_prefix(&[50]), None);
     /// assert_eq!(v.strip_prefix(&[10, 50]), None);
     ///
@@ -2620,7 +2624,8 @@ 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 `suffix` is empty, simply returns the original slice. If `suffix` is equal to the
+    /// original slice, returns an empty slice.
     ///
     /// If the slice does not end with `suffix`, returns `None`.
     ///
@@ -2630,6 +2635,7 @@ impl<T> [T] {
     /// let v = &[10, 40, 30];
     /// assert_eq!(v.strip_suffix(&[30]), Some(&[10, 40][..]));
     /// assert_eq!(v.strip_suffix(&[40, 30]), Some(&[10][..]));
+    /// assert_eq!(v.strip_suffix(&[10, 40, 30]), Some(&[][..]));
     /// assert_eq!(v.strip_suffix(&[50]), None);
     /// assert_eq!(v.strip_suffix(&[50, 30]), None);
     /// ```