doc: Mention the servent module.

[Collin Funk]

Hi,

I've applied the attached patch to mention the servent module in the
documentation for the functions getservbyname and getservbyport. These
functions exist on Windows in winsock2.h [1][2].

Also, since servent depends on sys_socket-h which will include
winsock2.h on Windows I am 99% sure the declarations will be included
correctly. I'll write a test for that soon though just to be sure.

Windows doesn't have endservent, getservent, and setservent though so I
won't mention it there.

Collin

[1] 
https://learn.microsoft.com/en-us/windows/win32/api/winsock2/nf-winsock2-getservbyname
[2] 
https://learn.microsoft.com/en-us/windows/win32/api/winsock2/nf-winsock2-getservbyport

0001-doc-Mention-the-servent-module.patch
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