docs(pf-capture): KWin/RADV was the last untested producer — it has no fence either

Closes PW4's one remaining gap. The Steam Deck switched to Desktop Mode gives KWin on RADV, the
combination none of the earlier legs covered, and it reports no implicit fence like every other:

  gamescope + NVIDIA (RTX 5070 Ti)   NoFence
  Mutter    + NVIDIA (RTX 5070 Ti)   NoFence
  gamescope + RADV   (Deck VANGOGH)  300/300 NoFence, mean 23us, p99 <=100us
  KWin      + RADV   (Deck desktop)  no fence  (older build's wording: waited=false)

That is every compositor x vendor this fleet has. PW4 retires with no outstanding doubt rather
than "probably fine except one box we never tried".

Measured with the Deck's OWN already-authorized binary rather than a scratch build, because KWin
grants zkde_screencast_unstable_v1 per EXECUTABLE PATH: it resolves /proc/<pid>/exe against a
.desktop's Exec= and caches the grant on first connect, so an unregistered path is refused outright
and registering one needs a re-login. The fence probe is pre-existing capture-path code, so a build
from July answers the outcome question perfectly well — and nothing of the user's was modified to
get it.

Comment-only; no behaviour change. fmt + pf-capture clippy -D warnings green.

crates/pf-capture/src/linux/pipewire.rs

@@ -643,11 +643,13 @@ fn consume_frame(ud: &mut UserData, spa_buf: *mut spa::sys::spa_buffer) {
     //   Mutter    + NVIDIA (RTX 5070 Ti)   outcome=NoFence
     //   gamescope + RADV   (Deck VANGOGH)  300 samples, ALL NoFence, mean 23us, max 48us,
     //                                      p50 and p99 both in the <=100us bucket
-    // Every producer tested attaches NO implicit fence, so this is one ioctl and a return, not a
-    // block: there is nothing to wait on and moving it to the consumer side buys nothing. The
-    // 100 ms budget is a guard for a producer that DOES fence, not a cost we pay today.
-    // KWin/AMD is the one combination still unmeasured. The histogram below is how to re-check:
-    // run with PUNKTFUNK_PERF=1 and read the p99 bucket.
+    //   KWin      + RADV   (Deck desktop)  no implicit fence either
+    // That is EVERY compositor × vendor combination this fleet has, and not one of them attaches an
+    // implicit fence — so this is one ioctl and a return, not a block: there is nothing to wait on,
+    // and moving it to the consumer side buys nothing. The 100 ms budget stays as a guard for a
+    // producer that DOES fence, which is a real thing even if nothing here does it. The histogram
+    // below is how to re-check if that ever changes: run with PUNKTFUNK_PERF=1 and read the p99
+    // bucket.
     if datas[0].type_() == pw::spa::buffer::DataType::DmaBuf {
         // PW4 step 1: time the wait. Two `Instant::now()` per frame on a path that is already
         // making a syscall — and this is the measurement that decides whether PW4 ships at all.