Remind to update zconf.h when upgrading zlib

And upgrade the submodule after performing the change documented here in
it to avoid link conflicts between the bundled zlib version and another
copy of zlib potentially used in the application due to the name clashes
between crc32_combine_xxx functions new in 1.2.12 that upstream forgot
to add to zconf.h.

Closes #22280.

docs/contributing/how-to-update-third-party-library.md

@@ -71,6 +71,15 @@ without `-DHAVE_CONFIG_H` as it works just fine without it on any ANSI C system
 
 Run `autogen.sh` to update the generated files and commit the changes.
 
+## zlib
+
+Check for the newly added public systems in `zlib.map` and update `zconf.h` to
+include if necessary -- at least with zlib 1.2.12 release this file wasn't
+updated at all in the upstream version, resulting in problems such as #22280.
+If `zconf.h` is updated, you probably already had to resolve the conflicts in
+it, as our file differs from the upstream version due to having the changes
+from [Z_PREFIX PR](https://github.com/madler/zlib/pull/323) included in it.
+
 
 Updating the main repository
 ----------------------------