update intro and overview sections of Multisig document

[namcios]

• minor grammar / typo fixes
• small suggestions on wording
• "

first world" is a bit outdated expression, adding "developed countries" as alternative
• Last Pass is not open source, recommending BitWarden instead because it is


[shannona]

thanks, @namcios. Good corrections (and I'd had a suspicion "first world" was now dated, so appreciate that fix).