docs(sidecar): put the measured read cost in the dismissal bound

sidecar/src/main/kotlin/dev/sanderling/sidecar/DriverBackend.kt

@@ -809,10 +809,12 @@ internal fun dismissSoftKeyboard(shell: (String) -> String) {
 }
 
 // KEYBOARD_DISMISS_READS bounds the re-reads a snapshot spends waiting for the
-// IME window to leave the tree after BACK. The window goes over an animation,
+// IME window to leave the tree after BACK. The window leaves over an
-// so the first read back can still carry it; a hierarchy read plus the interval
+// animation, so the first read back can still carry it. A hierarchy read
-// costs ~250ms on the emulator, which covers a retraction several times over
+// measures at a 76ms median and a 168ms p90 on the API 34 emulator, so with
-// without turning a keyboard the app keeps re-raising into an unbounded wait.
+// the interval these four reads watch most of a second: several retractions
+// over, without turning a keyboard the app keeps re-raising into a wait with
+// no end.
 internal const val KEYBOARD_DISMISS_READS = 4
 internal const val KEYBOARD_DISMISS_INTERVAL_MILLIS = 100L