DOC: Document the behavior of bar() for categorical x data

In particular, mention the different behavior on duplicates compared to using `tick_label` explicitly.

Closes matplotlib#29507.

Author: timhoffm

lib/matplotlib/axes/_axes.py

@@ -2386,6 +2386,17 @@ def bar(self, x, height, width=0.8, bottom=None, *, align="center",
             The x coordinates of the bars. See also *align* for the
             alignment of the bars to the coordinates.
 
+            Bars are often used for categorical data, i.e. string labels below
+            the bars. You can provide a list of strings directly to x.
+            ``bar(['A', 'B', 'C'], [1, 2, 3])`` is often a shorter and more
+            convenient notation compared to
+            ``bar(range(3), [1, 2, 3], tick_label=['A', 'B', 'C'])``. They are
+            equivalent as long as the names are unique. The explicit *tick_label*
+            notation draws the names in the sequence given. However, when having
+            duplicate values in categorical *x* data, these values map to the same
+            numerical x coordinate, and hence the corresponding bars are drawn on
+            top of each other.
+
         height : float or array-like
             The height(s) of the bars.