doc: add comment on method to set savepoint_limit dynamically savepoint_limit isn't a real config option as in config.ini. I generate one on the fly for use in the backend during import. While this is arguably a valid config option, it is also something that is niche (import a tracker to postgres) and adds clutter to config.ini.

# human timezones
pytz
# indexer
Whoosh
# xapian is also installed
# extra database support
psycopg2
mysqlclient
redis
# encryption
gpg
# java web tokens
PyJWT
# extra HTTP compression methods
Brotli
zstd
# jinja2 template engine
Jinja2