all: document every item and rework the comments

[karalabe]

Documents every item in the tool and its connection library, and reworks the comments to one consistent style.

• Every module, item, field, variant and test has a doc, including private and test code, local items and the winget packaging tool. The doc comments clap derives help text from are left as they are, so the ark help output is unchanged.
• Each doc opens with a one-sentence summary. Functions start with a verb, and types, fields and variants with a noun phrase. Named items are linked.
• Field and variant docs move from the end of the line to a doc comment above the item.
• Notes take one consistent shape, and bodies doing several things, tests included, read as commented steps. The Windows-only unsafe blocks gain soundness notes.
• Docs that contradicted the code now describe what it does, such as the failures a WebSocket upgrade error covers and when cached login headers are replaced.

Only comments and blank lines change.

It changes no code, so the code issues the pass noticed, such as a download test that cannot prove HTTPS enforcement, are left for separate changes.