Adjust documentation for some pointer-to-slice methods

This commit changes documentation of the following methods as proposed
by the PR review
- `<*const [T]>::len`
- `<*mut [T]>::len`
- `<*const str>::len`
- `<*mut str>::len`
- `<*const str>::get_unchecked`
- `<*mut str>::get_unchecked_mut`
- `NonNull::<str>::get_unchecked_mut`

library/core/src/ptr/const_ptr.rs

@@ -913,7 +913,7 @@ impl<T> *const [T] {
     /// The returned value is the number of **elements**, not the number of bytes.
     ///
     /// This function is safe, even when the raw slice cannot be cast to a slice
-    /// reference because the pointer is null or unaligned.
+    /// reference (e.g. because the pointer is null or unaligned).
     ///
     /// # Examples
     ///
@@ -1037,8 +1037,8 @@ impl *const str {
     ///
     /// The returned value is the number of **bytes**, not the number of characters.
     ///
-    /// This function is safe, even when the raw string slice cannot be cast to a slice
-    /// reference because the pointer is null or unaligned.
+    /// This function is safe, even when the raw string slice cannot be cast to a string slice
+    /// reference (e.g. because the pointer is null or unaligned).
     ///
     /// # Examples
     ///
@@ -1085,9 +1085,6 @@ impl *const str {
     ///
     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     ///
-    /// Note that calling this function with an index that does not lie on an UTF-8 sequence boundaries
-    /// is safe, but dereferencing the pointer returned by such call is unsound.
-    ///
     /// # Examples
     ///
     /// ```

library/core/src/ptr/mut_ptr.rs

@@ -1176,7 +1176,7 @@ impl<T> *mut [T] {
     /// The returned value is the number of **elements**, not the number of bytes.
     ///
     /// This function is safe, even when the raw slice cannot be cast to a slice
-    /// reference because the pointer is null or unaligned.
+    /// reference (e.g. because the pointer is null or unaligned).
     ///
     /// # Examples
     ///
@@ -1353,8 +1353,8 @@ impl *mut str {
     ///
     /// The returned value is the number of **bytes**, not the number of characters.
     ///
-    /// This function is safe, even when the raw string slice cannot be cast to a slice
-    /// reference because the pointer is null or unaligned.
+    /// This function is safe, even when the raw string slice cannot be cast to a string slice
+    /// reference (e.g. because the pointer is null or unaligned).
     ///
     /// # Examples
     ///
@@ -1406,9 +1406,6 @@ impl *mut str {
     ///
     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     ///
-    /// Note that calling this function with an index that does not lie on an UTF-8 sequence boundaries
-    /// is safe, but dereferencing the pointer returned by such call is unsound.
-    ///
     /// # Examples
     ///
     /// ```

library/core/src/ptr/non_null.rs

@@ -708,9 +708,6 @@ impl NonNull<str> {
     ///
     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     ///
-    /// Note that calling this function with an index that does not lie on an UTF-8 sequence boundaries
-    /// is safe, but dereferencing the pointer returned by such call is unsound.
-    ///
     /// # Examples
     ///
     /// ```