Changed FormatStrFormatter documentation to include how to get unicode minus (matplotlib#19665)

Author: The-Inky

lib/matplotlib/ticker.py

@@ -411,6 +411,10 @@ class FormatStrFormatter(Formatter):
 
     The format string should have a single variable format (%) in it.
     It will be applied to the value (not the position) of the tick.
+
+    Negative numeric values will use a dash not a unicode minus,
+    use mathtext to get a unicode minus by wrappping the format specifier
+    with $ (e.g. "$%g$").
     """
     def __init__(self, fmt):
         self.fmt = fmt