Add note on gmt 4 vector syntax

Author: yvonnefroehlich

examples/gallery/lines/vector_heads_tails.py

@@ -27,6 +27,11 @@
 the vector type, here ``"v"`` for Cartesian vector, see also the
 :doc:`Vector types example </gallery/lines/vector_styles>`)
 by passing the corresponding shortcuts to the ``style`` parameter.
+
+Note: Vectors were completely redesigned for GMT 5 which separated the vector
+head (a polygon) from the vector stem (a line). In GMT 4, the entire vector was
+a polygon. Thus, for plotting an arrow with a fill for both head and stem users
+have to follow the GMT 4 synthax explained at :gmt-docs:`plot.html#id13`.
 """
 
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