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.

try:
  # used for python2 and python 3 < 3.13
  import cgi as cgi
  from cgi import FieldStorage, MiniFieldStorage
except ImportError:
  # use for python3 >= 3.13
  from roundup.anypy.vendored import cgi
  from roundup.anypy.vendored.cgi import FieldStorage, MiniFieldStorage