🤔 Detail how sources wrap

[JFWooten4]

What problem does your feature solve? 📝

It is my present understanding of the Docs that there is not a source of reference detailing how operations can have a source account different than that of the parent transaction envelope. In re #1011, might we clarify explicitly how default source account overrides work for new users?

What would you like to see?

Explain how each transaction's operation can have it's own source, unique from an envelope source. 💌 Ideally, provide a few code examples showing some very basic applications. Materially, the signature implications ought be extensively detailed per key custody implications.

What alternatives are there? 💭

We could require new users to learn all this when they start messing around with the Lab, but that may not be ideal. Namely, a new developer could confuse transaction and operation source accounts in re set ops functions which lock an account, potentially through no fault of their own. 🔐 A brief and informative explanation here could greatly protect newcomers.