Multi-signing doc improvements

[mDuo13]

The multi-signing doc should talk more about the high-level concepts around multi-signing:

• Signer weights and quorum and how they relate
• High-level overview of how to use multi-signing, noting important stuff like how multi-signing adds a new way of authorizing transactions so if you want to be exclusively multi-signing you have to disable the ways of signing
• Maybe some more detailed descriptions of how you might accomplish some of the use cases described in the intro bullets

[mDuo13]

The multi-signing doc should also have examples using RippleAPI—for example, the combine() function—to collect and assemble multi-signatures

[q411]

https://xrpl.org/rippleapi-reference.html#combine

[mDuo13]

Duplicate of #685