Correct documentation for digest_algorithm and signing_algorithm

[naggie]

Description

The feature or problem addressed by this PR

The configuration options for digest_algorithm and signature_algorithm were incorrectly documented as an sp/idp option. See #405 for background.

What your changes do and why you chose this solution

Updated config.rst and changelog.

Checklist

• [x] Checked that no other issues or pull requests exist for the same issue/change
• [x] Added tests covering the new functionality (N/A)
• [x] Updated documentation OR the change is too minor to be documented
• [x] Updated CHANGELOG.md OR changes are insignificant