Updated expat update instructions.

Added change log.

CHANGES.rst

@@ -305,6 +305,8 @@ Other Changes and Additions
   manage the version number, and adding a ``pyproject.toml`` to opt in to
   isolated builds as described in PEP 517/518. [#9726]
 
+- Bundled ``expat`` is updated to version 2.2.9. [#10038]
+
 4.0.1 (unreleased)
 ==================
 

cextern/trim_expat.sh

@@ -4,10 +4,23 @@ set -euv
 
 # This script should be run every time expat is updated.
 
-# So, the standard update would be to execute, from this directory,
+# So, the standard update would be to execute, from this directory:
+#
 # rm -rf expat
 # tar xvf <PATH_TO_TAR>   # (e.g., expat-2.2.6.tar.bz2)
+# cd expat
+# ./buildconf.sh
+# ./configure
+# cp expat_config.h ../../astropy/utils/xml/src/
+# cd ..
 # ./trim_expat.sh
+# cd ../..
+# git add . -u
+#
+# And if the trim script missed anything, try to remove the extra files
+# with a "git clean -xdf" command, do a local build, and run a full test suite
+# with "pytest --remote-data" to make sure we didn't accidentally deleted an
+# important file.
 
 rm -rf expat/{conftools,doc,examples,m4,test,win32,CMake*,configure*,Makefile*,lib/*vcxproj*,tests,*.cmake,aclocal.m4,run.sh.in,test-driver-wrapper.sh,expat.*,xmlwf}