Most linux distributions either include C man pages by default or add them in when you install dev tools. I’d be surprised if man strchr didn’t already come up with something for you.

If you’re going down this path, it’s worth recognizing the different meanings of the man sections - shell commands in section 1 means that e.g. man chown is going to show you shell instead of the C function you’ll see from man 2 chown. In the same vein, section 3 has library functions, which are sometimes more relevant for you than section 2’s syscalls.