cli: document rebooting a reserved machine

The reserve command now defaults TMT_REBOOT_TIMEOUT to the reservation duration, so a reboot into an offline upgrade no longer costs the user their machine. Three cases still need the variable set by hand: a reservation extended with extend-reservation past the duration of the original request, a reservation made with request --reserve, which keeps the tmt default because the user's tests run first, and a request submitted straight to the API.

Depends on testing-farm/cli!244 (merged)

Resolves: TFT-5168 Assisted-by: Claude Code Signed-off-by: Miroslav Vadkerti mvadkert@redhat.com