docs: add API change notes

Author: kir0ul

doc/api/next_api_changes/development/20706-AP.rst

@@ -0,0 +1,4 @@
+Improve underscore support in LaTeX
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The `underscore <https://ctan.org/pkg/underscore>`_ package is now a requirement to improve support for underscores in LaTeX.

lib/matplotlib/texmanager.py

@@ -175,6 +175,7 @@ def _get_preamble(self):
             r"\usepackage[papersize=72in, margin=1in]{geometry}",
             self.get_custom_preamble(),
             # Use `underscore` package to take care of underscores in text
+            # The [strings] option allows to use underscores in file names
             r"\makeatletter"
             r"\@ifpackageloaded{underscore}{}"
             r"{\usepackage[strings]{underscore}}"