Add newer range patterns to unstable book

Author: workingjubilee

src/doc/unstable-book/src/language-features/exclusive-range-pattern.md

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+# `exclusive_range_pattern`
+
+The tracking issue for this feature is: [#37854].
+
+
+[#67264]: https://github.com/rust-lang/rust/issues/67264
+[#37854]: https://github.com/rust-lang/rust/issues/37854
+-----
+
+The `exclusive_range_pattern` feature allows non-inclusive range
+patterns (`0..10`) to be used in appropriate pattern matching
+contexts. It also can be combined with `#![feature(half_open_range_patterns]`
+to be able to use RangeTo patterns (`..10`).
+
+It also enabled RangeFrom patterns but that has since been
+stabilized.
+
+```rust
+#![feature(exclusive_range_pattern)]
+    match x {
+        0..10 => println!("single digit"),
+        10 => println!("ten isn't part of the above range"),
+        _ => println!("nor is everything else.")
+    }
+```

src/doc/unstable-book/src/language-features/half-open-range-patterns.md

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+# `half_open_range_patterns`
+
+The tracking issue for this feature is: [#67264]
+It is part of the `#![exclusive_range_pattern]` feature,
+tracked at [#37854].
+
+[#67264]: https://github.com/rust-lang/rust/issues/67264
+[#37854]: https://github.com/rust-lang/rust/issues/37854
+-----
+
+The `half_open_range_patterns` feature allows RangeTo patterns
+(`..10`) to be used in appropriate pattern matching contexts.
+This requires also enabling the `exclusive_range_pattern` feature.
+
+It also enabled RangeFrom patterns but that has since been
+stabilized.
+
+```rust
+#![feature(half_open_range_patterns)]
+#![feature(exclusive_range_pattern)]
+    match x {
+        ..10 => println!("got it"),
+        0 => println!("zilch")
+    }
+```