Improve HID keyboard documentation

Explain how to configure the keyboard layout.

README.md

@@ -803,6 +803,15 @@ of the host key mapping. Therefore, if your keyboard layout does not match, it
 must be configured on the Android device, in Settings → System → Languages and
 input → [Physical keyboard].
 
+This settings page can be started directly:
+
+```bash
+adb shell am start -a android.settings.HARD_KEYBOARD_SETTINGS
+```
+
+However, the option is only available when the HID keyboard is enabled (or when
+a physical keyboard is connected).
+
 [Physical keyboard]: https://github.com/Genymobile/scrcpy/pull/2632#issuecomment-923756915
 
 

app/scrcpy.1

@@ -90,6 +90,12 @@ This provides a better experience for IME users, and allows to generate non-ASCI
 
 It may only work over USB, and is currently only supported on Linux.
 
+The keyboard layout must be configured (once and for all) on the device, via Settings -> System -> Languages and input -> Physical keyboard. This settings page can be started directly:
+
+    adb shell am start -a android.settings.HARD_KEYBOARD_SETTINGS
+
+However, the option is only available when the HID keyboard is enabled (or a physical keyboard is connected).
+
 .TP
 .B \-\-legacy\-paste
 Inject computer clipboard text as a sequence of key events on Ctrl+v (like MOD+Shift+v).

app/src/cli.c

@@ -165,7 +165,14 @@ static const struct sc_option options[] = {
                 "generate non-ASCII characters, contrary to the default "
                 "injection method.\n"
                 "It may only work over USB, and is currently only supported "
-                "on Linux.",
+                "on Linux.\n"
+                "The keyboard layout must be configured (once and for all) on "
+                "the device, via Settings -> System -> Languages and input -> "
+                "Physical keyboard. This settings page can be started "
+                "directly: `adb shell am start -a "
+                "android.settings.HARD_KEYBOARD_SETTINGS`.\n"
+                "However, the option is only available when the HID keyboard "
+                "is enabled (or a physical keyboard is connected).",
     },
     {
         .shortopt = 'h',