Update docs

[stdtom]

Update docs to prevent confusion that exactly one of the arguments content,
sensitive_content, content_base64, and source needs to be specified.

Closes #116 and #122

[hashicorp-cla]

All committers have signed the CLA.

[detro]

Hello @stdtom - thank you so much for taking the time with this.

Would you mind adding a note for the next CHANGELOG entry, to specify the important documentation improvement you are introducing?

I think it should go under the NOTES: section.
And the next CHANGELOG entry should be for 2.2.3 I believe (being this a patch release).

[stdtom]

Hi @detro
I added a changelog entry - hope it meets your expextations.

[github-actions]

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