javadoc for base [disk,exceptions]

[jshook]

This is the first of the javadoc backfill PRs. This one is pretty light.
There will be 10-20 of these. Once we review/refine/merge them all, the codebase will be much better documented.

[github-actions]

Before you submit for review:

• Does your PR follow guidelines from CONTRIBUTIONS.md?
• Did you summarize what this PR does clearly and concisely?
• Did you include performance data for changes which may be performance impacting?
• Did you include useful docs for any user-facing changes or features?
• Did you include useful javadocs for developer oriented changes, explaining new concepts or key changes?
• Did you trigger and review regression testing results against the base branch via Run Bench Main?
• Did you adhere to the code formatting guidelines (TBD)
• Did you group your changes for easy review, providing meaningful descriptions for each commit?
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If you did not complete any of these, then please explain below.

[MarkWolters]

LGTM