Add an entry in the unstable book

Author: ricochet1k

src/doc/unstable-book/src/language-features/macro-lifetime-matcher.md

@@ -0,0 +1,14 @@
+# `macro_lifetime_matcher`
+
+The tracking issue for this feature is: [#46895]
+
+With this feature gate enabled, the [list of fragment specifiers][frags] gains one more entry:
+
+* `lifetime`: a lifetime. Examples: 'static, 'a.
+
+A `lifetime` variable may be followed by anything.
+
+[#46895]: https://github.com/rust-lang/rust/issues/46895
+[frags]: ../book/first-edition/macros.html#syntactic-requirements
+
+------------------------