Document address storage in AsyncFieldMixin. (pantsbuild#11448)

Without a note there is every reason to undo storage in AsyncFieldMixin
and push the address into Field in a refactoring commit.

src/python/pants/engine/target.py

@@ -227,6 +227,9 @@ def __init__(self, raw_value: Optional[Any], *, address: Address) -> None:
         # We must temporarily unfreeze the field, but then we refreeze to continue avoiding
         # subclasses from adding arbitrary fields.
         self._unfreeze_instance()  # type: ignore[attr-defined]
+        # N.B.: We store the address here and not in the Field base class, because the memory usage
+        # of storing this value in every field was shown to be excessive / lead to performance
+        # issues.
         self.address = address
         self._freeze_instance()  # type: ignore[attr-defined]