Document string_deref_patterns feature

Author: mejrs

src/doc/unstable-book/src/language-features/string-deref-patterns.md

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+# `string_deref_patterns`
+
+The tracking issue for this feature is: [#87121]
+
+[#87121]: https://github.com/rust-lang/rust/issues/87121
+
+------------------------
+
+This feature permits pattern matching `String` to `&str` through [its `Deref` implementation].
+
+```rust
+#![feature(string_deref_patterns)]
+
+pub enum Value {
+    String(String),
+    Number(u32),
+}
+
+pub fn is_it_the_answer(value: Value) -> bool {
+    match value {
+        Value::String("42") => true,
+        Value::Number(42) => true,
+        _ => false,
+    }
+}
+```
+
+Without this feature other constructs such as match guards have to be used.
+
+```rust
+# pub enum Value {
+#    String(String),
+#    Number(u32),
+# }
+#
+pub fn is_it_the_answer(value: Value) -> bool {
+    match value {
+        Value::String(s) if s == "42" => true,
+        Value::Number(42) => true,
+        _ => false,
+    }
+}
+```
+
+[its `Deref` implementation]: https://doc.rust-lang.org/std/string/struct.String.html#impl-Deref-for-String