After my little research :
The C POSIX library, which is the C standard library for POSIX systems, contains numerous additional functions to those of the ANSI C standard, most of them are dealing with the operating system for some extremely uses (multithreading implementation, networking(including any kind of communication(IPC for example))), users privileges manging, etc. )
The GNU/Linux operating system, for example, is compatible with the POSIX, therefore the glibc is implementing the C POSIX library standard(POSIX libc) which contains all its additional functions and headers with their Linux implementation using Linux's System Calls.
In our case, the documentation of sockets which introduce in Linux's manual page for developers belongs to the University of California, Berkeley (The developers of that form of sockets and modern networking in general). Because POSIX has "adopted" Berkeley's sockets and has included the header sys/socket.h in its C standard library the GNU/Linux system implements it either in its C standard library known as glibc.
Therefore, that sockets' documentation actually documents the glibc functions which are completely compatible with the POSIX's sockets functions which are compatible with the Berkeley's.
Thanks a lot for those who have helped!
BTW : Any other answers are welcome.