doc: Add developer note on c_str()
doc: Add developer note on c_str()

Add a note when to use and when not to use `c_str()`.


Former-commit-id: f71898c1de250d143c86002a97936ddb810a3ca7
syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin