Update LSIF upload warnings.
Created by: efritz
Remove some of the "coming soon to 3.10", rearranged data retention sections, and rephrased the warning that says not to upload more than 10-40 LSIF dumps (which is far too low). We've seen no impact on the dotcom instance running LSIF for two dozen repositories with ~300 active dumps (due to the default disk size).