[fenix] No issue: add kdoc to HomeActivity.

We could consider renaming the Activity to make it clearer that it's the
main activity and doesn't just feature the homescreen but I'm concerned
that renaming it will break too many things (e.g. automation that starts
a specific activity). For quick fix, I added this comment.

app/src/main/java/org/mozilla/fenix/HomeActivity.kt

@@ -72,6 +72,12 @@ import org.mozilla.fenix.theme.ThemeManager
 import org.mozilla.fenix.utils.BrowsersCache
 import org.mozilla.fenix.utils.RunWhenReadyQueue
 
+/**
+ * The main activity of the application. The application is primarily a single Activity (this one)
+ * with fragments switching out to display different views. The most important views shown here are the:
+ * - home screen
+ * - browser screen
+ */
 @SuppressWarnings("TooManyFunctions", "LargeClass")
 open class HomeActivity : LocaleAwareAppCompatActivity() {