docs(CHANGES) Document return type annotation fixes

why: Users relying on type hints should know about the corrected return types.
what:
- Add changelog entry for capture_pane() and display_message() type fixes

Author: tony

CHANGES

@@ -39,6 +39,8 @@ _Upcoming changes will be written here._
 - Normalize docs headings and Sphinx module directives to fix anchor and index generation issues.
 - Tweak Sphinx type-hints configuration to avoid RST indentation conflicts and suppress forward-reference warnings.
 - Refresh docstrings and cross-references (pane/window APIs, environment helpers, pytest plugin) for clearer return types and stable anchors.
+- Fix incorrect return type annotations for `capture_pane()` and `display_message()` methods
+  (changed from `str | list[str]` to `list[str]` - the methods always return a list).
 
 ## libtmux 0.50.0 (2025-11-30)