Document system_webview_package_name arg better

Improve the doc comment for the GN arg "system_webview_package_name" to
mention that it controls TrichromeWebView as well as SystemWebView, and
to explain more about why you might want to use it.

Change-Id: I4c4c6e003416c9e2f39ab2492078baaa049237b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575732
Commit-Queue: Richard Coles <torne@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652614}

android_webview/system_webview_apk_tmpl.gni

@@ -11,7 +11,10 @@ import("//chrome/android/trichrome.gni")
 import("//tools/v8_context_snapshot/v8_context_snapshot.gni")
 
 declare_args() {
-  # Package name of the system_webview_apk target.
+  # Android package name to use when compiling the system_webview_apk and
+  # trichrome_webview_apk targets. This should be used if the Android build
+  # on which you are going to install WebView is configured to load a
+  # different package name than the default used in AOSP.
   system_webview_package_name = "com.android.webview"
 }