[AW][Dev-UI] document autogenerated switches

No change to logic. This updates commandline-flags.md to explain
switches are now autogenerated with java_cpp_strings.

This is a follow-up to https://crrev.com/c/2343951.

Bug: 956803
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: Ic808e5301a57681246fd98910cf76c4f805b5f96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350392
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797068}

android_webview/docs/commandline-flags.md

@@ -141,9 +141,11 @@ Some interesting flags and Features:
 
 WebView also defines its own flags and Features:
 
- * [AwSwitches.java](https://cs.chromium.org/chromium/src/android_webview/java/src/org/chromium/android_webview/common/AwSwitches.java)
-   (and its [native
-   counterpart](https://cs.chromium.org/chromium/src/android_webview/common/aw_switches.h))
+ * C++ switches are defined in
+   [`aw_switches.cc`](/android_webview/common/aw_switches.cc). We use
+   [`java_cpp_strings`](/docs/android_accessing_cpp_switches_in_java.md) to
+   automatically generate Java switch constants from the C++ switches (see
+   [`AwSwitches.java`](https://source.chromium.org/chromium/chromium/src/+/master:out/android-Debug/gen/android_webview/common_java/generated_java/input_srcjars/org/chromium/android_webview/common/AwSwitches.java)).
  * [AwFeatureList.java](https://cs.chromium.org/chromium/src/android_webview/java/src/org/chromium/android_webview/AwFeatureList.java)
    (and its [native
    counterpart](https://cs.chromium.org/chromium/src/android_webview/common/aw_features.h))