Change docs for walletcreatefundedpsbt RPC method

Summary:

sequence field in the list of inputs currently marked as "required". Actually it can be omitted and it's value depends on locktime and options.replaceable fields. Just the same as in createpsbt call.

This is a backport of core#19018

Test Plan:

src/bitcoin-cli help walletcreatefundedpsbt

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9940