Better Definitions for Mixed Parameters and Values Part 2 of Many

[oleibman]

Continuing work started with PR #4016. Improve documentation within program by making explicit what types of values are allowed for variables described as "mixed". In order to avoid broken functionality, this is done mainly through doc-blocks. This will get us closer to Phpstan Level 9, but many changes will be needed before we can consider that.

This change has more executable code changes than its predecessor. I will wait longer than normal before merging it to allow for additional testing.

This is:

• [ ] a bugfix
• [ ] a new feature
• [ ] refactoring
• [ ] additional unit tests
• [x] code documentation

Checklist:

• [x] Changes are covered by unit tests
  • [x] Changes are covered by existing unit tests
  • [x] New unit tests have been added
• [x] Code style is respected
• [x] Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• [ ] CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• [ ] Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate). If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.