Document post-release Formula and navigation follow-ups

[s243a]

Summary

Records the post-release UI follow-ups discussed during v1.3.1 / v1.1.1 testing:

• an explicit, duplicate-safe SymPy setup-cell helper;
• touch-, keyboard-, and screen-reader-accessible Formula usage help;
• Find and Help as tri-state header shortcuts, defaulting to When there is room, with permanent main-menu fallbacks;
• parent-aware submenu Back behavior; and
• a possible future tabbed Help/About layout.

This is documentation only. It changes no shipped behavior and is explicitly non-blocking for the current release.