Fix and update flags documentation

[mzdravkov]

Fix and update the documentation for the functions that query the flag field

Types of changes

No functional changes, only updates in the documentation.

• Add a section in the main documentation page for the functions in flags.jl. They already had docstrings, but they were not exposed in the web page, making it harder for people to know they exist.
• A section for the flags.jl functions in the API page.
• A few of the docstrings in flags.jl where referring to old names of the functions that have since been changed.
• I'm not sure if I am running the doc generation incorrectly, but I get errors because the docs for some functions are duplicated (they're both in the main index page and in API) as well as some broken links to opencollective. I added warnonly in makedocs to make such issues non-breaking, but if that's not the proper way to do it, please let me know and I can change it.

☑️ Checklist

• 🎨 The changes implemented is consistent with the julia style guide.
• 📘 I have updated and added relevant docstrings, in a manner consistent with the documentation styleguide.
• 📘 I have added or updated relevant user and developer manuals/documentation in docs/src/.
• 🆗 There are unit tests that cover the code changes I have made.
• 🆗 The unit tests cover my code changes AND they pass.
• 📝 I have added an entry to the [UNRELEASED] section of the manually curated CHANGELOG.md file for this repository.
• 🆗 All changes should be compatible with the latest stable version of Julia.
• 💭 I have commented liberally for any complex pieces of internal code.