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.

#! /bin/sh

# if there are multiple trackers, d=demo t=tracker ...
# returns last one for testing that server is up. Does not test
# each tracker.
tracker=$(ps -ef | sed -ne '/roundup-server/s/^.*\s\(\w*\)=.*$/\1/p')
wget -q -O /dev/null --proxy off --no-verbose http://localhost:8080/"${tracker:-demo}"/