docs: complete Arch prerequisites and scope device setup notes

Document matching kernel headers, pacman dependencies, repository checkout, mode-switch destination creation and MCU1 branch selection. Distinguish missing DKMS builds from stale module indexes and firmware-loader IDs from working interfaces. Correct the installer support claim and explain the NetworkManager connectivity penalty with its upstream reference.

Keeps the contributor-reported Lukton DQ-211 results scoped to that unit. Validation: all Bash examples parse, local links resolve, and commands were checked against the repository installer, Makefile, AUR packaging and upstream documentation. Follow-up corrections for #109; no Arch hardware test is claimed.

(cherry picked from commit 1b071be3e1)

README.md

@@ -98,8 +98,8 @@ must pass the wireless stack version separately.
 
 For Arch-based distributions (Arch, CachyOS, …), see
 [Arch Linux / CachyOS install notes](INSTALL_ARCH.md) — including the AUR/DKMS
-path, the `/etc/usb_modeswitch.d/1111:1111` file the AUR package does not ship,
-and the `depmod` step needed after DKMS installation.
+path, matching kernel headers, mode-switch configuration, and module-index
+troubleshooting.
 
 #### Method 1: [Quick Installation](INSTALL_SCRIPT.md) (Recommended)