Help users support sending MPP keysend

Help users support sending MPP keysend

When routing a keysend payment, the user may want to signal to the
router whether to find multi-path routes in the
`PaymentParameters::for_keysend` helper, without going through manual
construction. Since some implementations do not support MPP keysend, we
have the user make the choice here rather than making it the default.

Some implementations will reject keysend payments with payment secrets,
so this commit also adds docs to `RecipientOnionFields` to communicate
this to the user.