Add a behaviour change note

Author: dstansby

doc/api/next_api_changes/behavior/25061-DS.rst

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+TwoSlopeNorm now auto-expands to always have two slopes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In the case where either ``vmin`` or ``vmax`` are not manually specified
+to `~.TwoSlopeNorm`, and where the data it is scaling is all less than or
+greater than the center point, the limits are now auto-expanded so there
+are two symmetrically sized slopes either side of the center point.
+
+Previously ``vmin`` and ``vmax`` were clipped at the center point, which
+caused issues when displaying color bars.
+
+This does not affect behaviour when ``vmin`` and ``vmax`` are manually
+specified by the user.