There's no "resource hub".
If you are talking about headers defined by various standards (C standard, POSIX standard, etc.) then standards bodies provide standard documents that serve as a reference; and there are plenty of third-party avenues that provide documentation (e.g. Linux man pages).
If you mean any particular project's API (let's say MySQL to pick a random example). Typically , the project should provide added documentation that describes how to do it , along with a description of all functions in reference format.
Sometimes, the project's developer might not have written documentation, but they may have put good comments within the header files that describe the expected inputs and outputs of every functions, and possibly even a guide on how to get started.
Programmers are often poor at writing documentation.. it's a skill to learn like any other. An example of terrible documentation would be OpenSSL. (I don't mean to offend any developers who put in a lot of hard work on the product , I just refer to the available documentation and readability of header files).