doc: Unify style in config support info

This builds on b31d6b7 (#1531) by adjusting the capitalization and
quoting style of string arguments in `progress.rs` that help
document the status of what configuration options are and are not
planned.

- Since I believe these strings are not usually rendered as
  Markdown, and most places that code formatting in Markdown would
  be used were already written with single quotes, this applies
  that quoting style consistently.

- This applies initial capitalization, which was sometimes but not
  always done. It does not apply a consistent style for when a
  string will end in a `.`, which seems already to differ
  deliberately for reasons of clarity. That can be adjusted later.

- A small amount of minor rewording for clarity is also included.

Author: EliahKagan