Explain shell is something that has helped me a lot, but the situation of needing it left me baffled for so long... Why can't I search by command + flag? For example, I know what command A does, but I'm not sure what flag C means. I can search for man A, but why can't I search man A --flag C to get only the relevan portion of the manpage?

This has baffled me for years. I'd have guessed that discoverability and documentation would be a core functionality and focus of open source developers.

Also, if I find a command which flag is not explained by explain shell, is there some source I can create PR to? Some DB, where I could propose the addition..?

Then you only get the line with "bar" in it. (append -C 1 at the end to get 1 line before and 1 after the term you grepped for).

And of course you can pimp your terminal experience by using fish (https://fishshell.com/) as it ships with autosuggestions and other really nice features. (Beware that it will not provide 100% POSIX compliance but if that's an issue you can use ZSH or pimp your bash as well).

Then there is other kind of man pages like TLDR (https://github.com/tldr-pages/tldr) which is curated by community and quite awesome ;-)