[3.5] Document dart pub downgrade --tighten (dart-lang#5807)

Landed in dart-lang/pub#4245

Probably won't be out until Dart 3.5

---------

Co-authored-by: Parker Lougheed <parlough@gmail.com>

Author: 2 people

src/content/tools/pub/cmd/pub-downgrade.md

@@ -111,5 +111,14 @@ For options that apply to all pub commands, see
 
 Reports what dependencies would change but doesn't change any.
 
+### `--tighten`
+
+:::version-note
+Support for the `--tighten` option is set to be added in Dart 3.5.
+:::
+
+Updates the lower bounds of dependencies in `pubspec.yaml` to match the
+resolved versions, and returns a list of the changed constraints. 
+Can be applied to [specific dependencies](#downgrading-specific-dependencies).
 
 {% render 'pub-problems.md' %}