Note greedy string arguments in docs

[SPGoding]

Purpose

Make it clear what should be put for some greedy string arguments.

Approach

Future work

Checklist

• [ ] Included tests
• [x] Updated documentation in README and docs folder
• [ ] Tested in MCTEST-xxx

[Pokechu22]

It might be useful to explicitly note something like "copy the username from the username field, not the URL" (since that's where the pluses came from).