doc: Add mention of BIP125 used by wallet GUI by default since v0.18.1
doc: Add mention of BIP125 used by wallet GUI by default since v0.18.1

Former-commit-id: bf0e619acfe1fc55791cbc97525960edc6d62d1a
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin