Add entry for `const_if_match` in unstable book

diff --git a/src/doc/unstable-book/src/language-features/const-if-match.md b/src/doc/unstable-book/src/language-features/const-if-match.md
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+# `const_if_match`
+
+The tracking issue for this feature is: [#49146]
+
+[#49146]: https://github.com/rust-lang/rust/issues/49146
+
+------------------------
+
+Allows for the use of conditionals (`if` and `match`) in a const context.
+Const contexts include `static`, `static mut`, `const`, `const fn`, const
+generics, and array initializers. Enabling this feature flag will also make
+`&&` and `||` function normally in a const-context by removing the hack that
+replaces them with their non-short-circuiting equivalents, `&` and `|`, in a
+`const` or `static`.