Added more reference docs to backend

[onel]

Added reference documentation to the following files:

• src/backend/src/modules/puterfs/DatabaseFSEntryFetcher.js
• src/backend/src/modules/dns/DNSService.js
• src/backend/src/filesystem/ll_operations/ll_write.js
• src/backend/src/filesystem/strategies/storage_a/LocalDiskStorageStrategy.js
• src/backend/src/om/entitystorage/WriteByOwnerOnlyES.js

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[jelveh]

Hi @onel, thanks for the PR! Could you please look into the CLA before we can review and merge?

[onel]

I'm pretty sure I signed it because I've contributed in the past.
Could it be that it shows the error because the commits are signed by the askmanu app instead of me personally?