[CIVIS-1509] DOC improve documentation of input to `civis.io.civis_file_to_table`

[Drew-Pappas]

This PR makes explicit the implicit requirement that Civis files in civis.io.civis_file_to_table must be in CSV-like format.

Closes #422

• [x] (For Civis employees only) Reference to a relevant ticket in the pull request title
• [x] Changelog entry added to CHANGELOG.md at the repo's root level
• [x] Description of change in the pull request description
• [x] If applicable, unit tests have been added and/or updated
• [x] The CircleCI builds have all passed

Screenshot of changes: