Fix docs for LSIF option DBS_DIR_MAXIMUM_SIZE_BYTES

Created by: dadlerj

Reported by https://app.hubspot.com/contacts/2762526/company/2931649618

Our docs (https://docs.sourcegraph.com/user/code_intelligence/lsif#data-retention-policy) say:

The bulk of LSIF data is stored on-disk, and as code intelligence data for a commit ages it becomes less useful. Sourcegraph will automatically remove the least recently uploaded data if the amount of disk space falls below a configurable threshold. This value defaults to 10 GiB (10⨉2^30 = 10737418240 bytes), and can be changed via the DBS_DIR_MAXIMUM_SIZE_BYTES environment variable.

But this is incorrect. The DBS_DIR_MAXIMUM_SIZE_BYTES reflects the maximum size of uploads on disk, not a minimum amount of extra space required (as the bolded words above indicate).