document :vis in unstable book

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

@@ -114,6 +114,7 @@
 - [lookup_host](lookup-host.md)
 - [loop_break_value](loop-break-value.md)
 - [macro_reexport](macro-reexport.md)
+- [macro_vis_matcher](macro-vis-matcher.md)
 - [main](main.md)
 - [manually_drop](manually-drop.md)
 - [map_entry_recover_keys](map-entry-recover-keys.md)

src/doc/unstable-book/src/macro-vis-matcher.md

@@ -0,0 +1,14 @@
+# `macro_vis_matcher`
+
+The tracking issue for this feature is: [#41022]
+
+With this feature gate enabled, the [list of fragment specifiers][frags] gains one more entry:
+
+* `vis`: a visibility qualifier. Examples: nothing (default visibility); `pub`; `pub(crate)`.
+
+A `vis` variable may be followed by a comma, ident, type, or path.
+
+[#41022]: https://github.com/rust-lang/rust/issues/41022
+[frags]: ../book/first-edition/macros.html#syntactic-requirements
+
+------------------------