For fenix#17451: add comment warning about Log.Priority.VERBOSE.

The reason we did this is mentioned in
mozilla-mobile#9451 (comment)
as:

> To limit the options to an accepted subset. (We also did not add the
> ASSERT level).

For a satisfying answer, I think we'd need to ask "Why?" again. However,
I don't want to spend more time on this so I didn't follow up on it. This
is why I didn't mention any reasoning for this in the code comment.

components/support/base/src/main/java/mozilla/components/support/base/log/Log.kt

@@ -100,6 +100,9 @@ object Log {
     enum class Priority(val value: Int) {
         // For simplicity the values mirror the Android log constants values:
         // https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/core/java/android/util/Log.java
+        //
+        // We intentionally omit ASSERT and VERBOSE. If you change this,
+        // be aware of the impact on consumers.
 
         DEBUG(android.util.Log.DEBUG),
         INFO(android.util.Log.INFO),