docs(CHANGES): Note doctest PR

Author: tony

CHANGES

@@ -22,6 +22,8 @@ $ pipx install --suffix=@next 'tmuxp' --pip-args '\--pre' --force
 ### Internal
 
 - libtmux updated from v0.12 to v0.14 {issue}`790`
+- Add [doctest](https://docs.python.org/3/library/doctest.html) w/
+  [pytest + doctest](https://docs.pytest.org/en/7.1.x/how-to/doctest.html), ({issue}`791`).
 
 ## tmuxp 1.12.1 (2022-08-04)