Give extra its proper Any typing, handle None vs. 0 vs. empty string gracefully in default formatter.

docs/database.md

@@ -212,9 +212,10 @@ def resolve_user_key(value: str) -> str | None:
 
 - By default a transaction is logged with an `action by user` header followed
   by the diff lines.  Added paths are colored green, deleted paths red.
-- `kanta.transaction(..., extra="...")` accepts a display-only string that is
-  appended after the action in the header (colored by Kanta); it is never
-  persisted in the `ChangeRecord`.
+- `kanta.transaction(..., extra=...)` accepts a display-only value that is
+  shown after the action in the header.  Anything other than `None` is
+  printed str-converted (colored by Kanta), unless a custom logemit handler
+  does something else with it; it is never persisted in the `ChangeRecord`.
 - `kanta.transaction(..., logdiff=False)` skips building and printing the diff
   body and logs only the header, which is useful for large or noisy
   changesets.  Diff output can also be disabled globally with