docs(niri): find a keysym without a tool that is not installed

`key-bindings-syntax.md` told you to run `wev` and read `sym: XXXX` — a placeholder nobody
fills in, for a tool that is not on the machine: `command -v wev` is empty, `pacman -Ss
--sync wev` is empty on CachyOS, and the AUR carries it as `wev-git`. The bind token is an
XKB keysym, and the complete list is already on every box that links libxkbcommon, so the
page now leads with the grep against `/usr/include/xkbcommon/xkbcommon-keysyms.h` (run here:
`a A Escape Left Return Tab`), and keeps `wev` as the "press the key and see" option with its
real package name and where to get it.

`screenshots.md` line 35 read `(FIXME upstream).`, which scans as unfinished work in a page
that is finished: the defect is upstream's, tracked in the two issue links six lines below.
Reworded to say that.

Refs #64

omp-docs/tools/niri/screenshots.md

@@ -32,7 +32,7 @@ Noctalia settings lives on `Mod+Shift+O` (moved off Mod+Shift+S).
 
 - Portal chain: flameshot 14 → `xdg-desktop-portal-gnome` → niri D-Bus
   API → `screenshot_all_outputs` → `ensure!(outputs.len() == 1)`
-  (FIXME upstream).
+  (an upstream defect, still unfixed — issue links below).
 - `flameshot full` on 2 outputs: each press emits **3 refusals**
   (composited screen + per-output) — `WARN niri::niri: error taking a
   screenshot: Condition failed: outputs.len() == 1 (2 vs 1)` in