Update -blocksonly documentation

Summary:

When -blocksonly is set to 1, it interacts with the -walletbroadcast
parameter and sets it to 0 if it has not been set already.This behavior
is not captured by the current documentation, which claims that -blocksonly
does not impact any wallet transactions.

Update the max number of outgoing peers from 8 to 10, due to the
addition of two -blocksonly peers.

This is a backport of Core PR18391 and PR18464

Test Plan: ninja && src/bitcoind -help

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8894