Add whats new for MarkerStyle features

Author: deep-jkl

doc/users/next_whats_new/extending_MarkerStyle.rst

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+New customization of MarkerStyle
+--------------------------------
+
+New MarkerStyle parameters allow control of join style and cap style. 
+The appearance of individual markers can be further controlled by 
+transform supplied during creation.
+
+.. plot::
+    :include-source:
+    import matplotlib.pyplot as plt
+    from matplotlib.markers import MarkerStyle
+    from matplotlib.transforms import Affine2D
+    fig, ax = plt.subplots(figsize=(6, 1))
+    fig.suptitle('New markers', fontsize=14)
+    for col, (size, rot) in enumerate(zip([2, 5, 10], [0, 45, 90])):
+        t = Affine2D().rotate_deg(rot).scale(size)
+        ax.plot(col, 0, marker=MarkerStyle("*", transform=t))
+    ax.axis("off")
+    ax.set_xlim(-0.1, 2.4)