sfdc api username and field security clarifications

[maximum-pixels]

What do these changes do?

Turns out SFDC's API can't handle a + in the username, causing an auth error, so included a warning for that. Also include a FAQ item around getting errors for fields not existing even when they do, due to not being visible to the SFDC user used in RS config.

Type of documentation

• [ ] New documentation
• [x] Documentation update
• [ ] Hotfix

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[ameypv-rudder]

thanks @maximum-pixels for your PR! We've added it to our queue for the week.

[ameypv-rudder]

@utsabc do you have any suggestions on the above updates? If no, we can proceed to merge this PR.

[ameypv-rudder]

@maximum-pixels please let me know if we're good to merge this one.