docs(pain-control): remove incorrect case-insensitivity note

[MattKotsenas]

The NOTE claims psmux treats key bindings case-insensitively (H == h), but psmux binding lookup is case-sensitive: in psmux core (psmux/psmux), normalize_key_for_binding in src/config.rs strips Shift while preserving character case, so h and H resolve to distinct bindings.

The note uses that false premise to justify the Alt-based resize keys; the section header (# PANE RESIZING (Alt + vim keys)) already documents the Alt keys, so removing the note loses nothing.

[psmux]

Hey @MattKotsenas, thanks for catching this. I went and checked it two ways before merging.

First, in psmux core itself: normalize_key_for_binding in src/config.rs (around line 1433) only ever strips KeyModifiers::SHIFT from the modifier set. It never touches key.0, so a KeyCode::Char('h') and a KeyCode::Char('H') stay two completely different values all the way through binding resolution. There is no lowercasing anywhere in that path.

Second, I proved it live rather than trusting the read. On an isolated psmux -L reviewtest server I bound h and H in the prefix table to two different set-option calls, then drove real keystrokes into the console with a WriteConsoleInput injector (CLI send-keys bypasses the client's key dispatch entirely, so it will not exercise this path, worth remembering for future tests). prefix+h set the option to lower-h, and prefix+H (Shift held) set it to upper-H. Two distinct results, so the old NOTE was simply wrong: psmux does not treat bindings case-insensitively.

Merged as c5de07f. Appreciate you tightening up the docs.