This patch:
1. Removes hardcoded mentions of a `--json` flag from the
documentation for JSONSchemaVersion, because the type could be used
for anything.
2. Removes `code` formatting because Go doc comments don’t support
this syntax.
3. Fixes [links] in doc comments so they link to the types’
online documentation.
4. Checks that JSONSchemaVersion satisfies the flag.Value interface.
5. Adds documentation examples for using both JSONSchemaVersion and
ResponseEnvelope.
Updates #17613
Updates #18750
Signed-off-by: Simon Law <sfllaw@tailscale.com>