Tweak RuntimeInit docs around MimeMap registration.

Bug: 136256059
Test: Treehugger

Change-Id: I3f82a1684f47d96630badef4c13b3f507f30eaa5

core/java/com/android/internal/os/RuntimeInit.java

@@ -214,7 +214,10 @@ public class RuntimeInit {
         RuntimeHooks.setTimeZoneIdSupplier(() -> SystemProperties.get("persist.sys.timezone"));
 
         /*
-         * Set a default mapping between MIME types and file extensions.
+         * Replace libcore's minimal default mapping between MIME types and file
+         * extensions with a mapping that's suitable for Android. Android's mapping
+         * contains many more entries that are derived from IANA registrations but
+         * with several customizations (extensions, overrides).
          */
         MimeMap.setDefault(MimeMapImpl.createDefaultInstance());