DOC: annotation tightbbox API change

Author: brunobeltran

doc/api/next_api_changes/behavior/18772-BGB.rst

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+Annotations with ``annotation_clip`` no longer affect ``tight_layout``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Previously, `.text.Annotation.get_tightbbox` always returned the full
+`.text.Annotation.get_window_extent` of the object, independent of the value
+of ``annotation_clip``. `.text.Annotation.get_tightbbox` now correctly takes
+this extra clipping box into account, meaning that `~.text.Annotation`\s that
+are not drawn because of ``annotation_clip`` will not count towards the axes
+bounding box calculations, such as those done by `~.pyplot.tight_layout`.
+
+This is now consistent with the API described in `~.artist.Artist`, which
+specifies that ``get_window_extent`` should return the full extents and
+``get_tightbbox`` should "account for any clipping".