document where to find ios device names (fix #3411)

docs/en/writing-running-appium/caps.md

@@ -7,7 +7,7 @@
 |`automationName`|Which automation engine to use|`Appium` (default) or `Selendroid`|
 |`platformName`|Which mobile OS platform to use|`iOS`, `Android`, or `FirefoxOS`|
 |`platformVersion`|Mobile OS version|e.g., `7.1`, `4.4`|
-|`deviceName`|The kind of mobile device or emulator to use|`iPhone Simulator`, `iPad Simulator`, `iPhone Retina 4-inch`, `Android Emulator`, `Galaxy S4`, etc...|
+|`deviceName`|The kind of mobile device or emulator to use|`iPhone Simulator`, `iPad Simulator`, `iPhone Retina 4-inch`, `Android Emulator`, `Galaxy S4`, etc.... On iOS, this should be one of the valid devices returned by instruments with `instruments -s devices`. On Android this capability is currently ignored.|
 |`app`|The absolute local path _or_ remote http URL to an `.ipa` or `.apk` file, or a `.zip` containing one of these. Appium will attempt to install this app binary on the appropriate device first. Note that this capability is not required for Android if you specify `appPackage` and `appActivity` capabilities (see below). Incompatible with `browserName`.|`/abs/path/to/my.apk` or `http://myapp.com/app.ipa`|
 |`browserName`|Name of mobile web browser to automate. Should be an empty string if automating an app instead.|'Safari' for iOS and 'Chrome', 'Chromium', or 'Browser' for Android|
 |`newCommandTimeout`|How long (in seconds) Appium will wait for a new command from the client before assuming the client quit and ending the session|e.g. `60`|