RPC: Document "payment_addr" usage

[C-Otto]

The protocol documentation includes a field payment_addr for AddInvoice:
https://api.lightning.community/#addinvoice

The documentation reads

The payment address of this invoice. This value will be used in MPP payments, and also for newer invoies [sic] that always require the MPP paylaod [sic] for added end-to-end security.

While this might explain the need for the field, it does not tell me how to populate the field when I create a new invoice. The code seems to generate some random bytes for this (see https://github.com/lightningnetwork/lnd/blob/master/lnrpc/invoicesrpc/addinvoice.go#L313), and the data provided via the RPC call seems to be ignored (see https://github.com/lightningnetwork/lnd/blob/master/rpcserver.go#L4739).

Please fix the documentation so that RPC users have an idea what kind of data to provide there.

Related:

• invoices/invoiceregistry: generate random payaddr for keysend #4343
• features+invoices: force MPP payload inclusion for non-keysend payments #4752
• Flag for mandatory payment_addr #4661
• channeldb: index payments by payment addr, use payment hash as fallback #4285

Your environment

• version of lnd: commit=v0.11.0-beta-509-g7f9f4a7c8d46962b9f4a6e2fbec716c6511fdd97
• which operating system (uname -a on *Nix): Linux ftp 5.8.0-0.bpo.2-amd64 Fix name typo in README #1 SMP Debian 5.8.10-1~bpo10+1 (2020-09-26) x86_64 GNU/Linux
• version of btcd, bitcoind, or other backend: Bitcoin Core 0.20.0

[C-Otto]

Superseeded by #4893 (as the "payment_addr" field is not supposed to be an input to the add invoice call).