How to find man pages for C structs (struct sockaddr_in)?
You can find that specific structure by typing man 7 ip and then searching manually for it or pressing / and typing sockaddr_in.
If you don't know what section to look in you can try using a quick search with man -k sockaddr_in. If that doesn't give you the result you are looking for try brute-forcing it with man -K sockaddr_in, this might be very slow because it will scan all the man files on your system.
@Jean-FrançoisFabre his comment about using a search engine is probably the fastest way to find the information you want though.