chore(iOS): add code comment explaining validity of a tag value (#2840)

## Description

Following up:

*
#2249 (comment)
* [internal
ticket](https://github.com/orgs/software-mansion/projects/3/views/1?pane=issue&itemId=72358100)

## Changes

Added a code comment explaining that there are scenarios where
`self.tag` might not have
valid value during component removal from screen stack.

## Test code and steps to reproduce

N/A

## Checklist

- [ ] Ensured that CI passes

Author: kkafar

ios/RNSScreenStack.mm

@@ -1234,6 +1234,9 @@ - (void)mountingTransactionDidMount:(const facebook::react::MountingTransaction
                withSurfaceTelemetry:(const facebook::react::SurfaceTelemetry &)surfaceTelemetry
 {
   for (const auto &mutation : transaction.getMutations()) {
+    // Note that self.tag might be invalid in cases this stack is removed.
+    // This mostlikely does not cause any problems now, but it is something
+    // worth to be aware of.
     if (MUTATION_PARENT_TAG(mutation) == self.tag &&
         (mutation.type == react::ShadowViewMutation::Type::Insert ||
          mutation.type == react::ShadowViewMutation::Type::Remove)) {