Mention stay awake limitation

The "stay awake" feature only works when the device is plugged in.

Refs #1445 <https://github.com/Genymobile/scrcpy/issues/1445>

README.md

@@ -417,7 +417,7 @@ The secondary display may only be controlled if the device runs at least Android
 
 #### Stay awake
 
-To prevent the device to sleep after some delay:
+To prevent the device to sleep after some delay when the device is plugged in:
 
 ```bash
 scrcpy --stay-awake

app/scrcpy.1

@@ -167,7 +167,7 @@ Default is "info" for release builds, "debug" for debug builds.
 
 .TP
 .B \-w, \-\-stay-awake
-Keep the device on while scrcpy is running.
+Keep the device on while scrcpy is running, when the device is plugged in.
 
 .TP
 .B \-\-window\-borderless

app/src/cli.c

@@ -159,7 +159,8 @@ scrcpy_print_usage(const char *arg0) {
 #endif
         "\n"
         "    -w, --stay-awake\n"
-        "        Keep the device on while scrcpy is running.\n"
+        "        Keep the device on while scrcpy is running, when the device\n"
+        "        is plugged in.\n"
         "\n"
         "    --window-borderless\n"
         "        Disable window decorations (display borderless window).\n"