Add splice to the unstable book.

Author: mattico

src/doc/unstable-book/src/SUMMARY.md

@@ -195,6 +195,7 @@
     - [slice_rsplit](library-features/slice-rsplit.md)
     - [sort_internals](library-features/sort-internals.md)
     - [sort_unstable](library-features/sort-unstable.md)
+    - [splice](library-features/splice.md)
     - [step_by](library-features/step-by.md)
     - [step_trait](library-features/step-trait.md)
     - [str_checked_slicing](library-features/str-checked-slicing.md)

src/doc/unstable-book/src/library-features/splice.md

@@ -0,0 +1,24 @@
+# `splice`
+
+The tracking issue for this feature is: [#32310]
+
+[#32310]: https://github.com/rust-lang/rust/issues/32310
+
+------------------------
+
+The `splice()` method on `Vec` and `String` allows you to replace a range
+of values in a vector or string with another range of values, and returns
+the replaced values.
+
+A simple example:
+
+```rust
+#![feature(splice)]
+let mut s = String::from("α is alpha, β is beta");
+let beta_offset = s.find('β').unwrap_or(s.len());
+
+// Replace the range up until the β from the string
+let t: String = s.splice(..beta_offset, "Α is capital alpha; ").collect();
+assert_eq!(t, "α is alpha, ");
+assert_eq!(s, "Α is capital alpha; β is beta");
+```