I don't need to read C++ docs, I am not concerned with it. If C++ is part of your, what's it called, stack, read the part of it that matters for your use case and read those parts directly related to your part of C++ or whatever other language you are using. Do that for every other language in your stack. I never said anything else, did I?
How deep and broad your understanding has to be depends on:
- your team, nobody can know everything alone, it is a team sport
- your specific use case, I cannot help you with that
- complexity of your use case
- your role, if you are responsible for graphics under Linux, sure as hell you read that part of the Linux documentation, front to end, multiple times and master it
- and, as always, know your system, stack, tools and documentation well enough to realize when you have to look stuff up and where