docs: `borg serve` does not inform of confidentiality implications

[Ofenhed]

The documentation of borg serve (before borg 2's --permissions) is too vague. While never clearly stated, it's not unreasonable to assume that --append-only implies write only. As this is a really dangerous assumption to make, it should be clearly stated that borg serve gives full read access to the entire repository it serves.

[ThomasWaldmann]

Well, append-only means it is never modifying old segment files, but only appends new transactions at the end (in new segment files). That property makes it possible to roll back a transaction by just deleting the segment files "at the end".

The opposite of append-only would be to (also) touch previously written segment files. Borg usually does not do that anyway, but there is one exception: borg compact, which needs to do that (and which is disabled in append-only mode).

Feel free to make a PR against 1.4-maint branch with a clarification.

Before modifying docs files, check their 1st line if they say they are auto-generated - do not touch these.

[ThomasWaldmann]

See also: #9270