Use standard nomenclature for UNIX domain sockets

[sorear]

UNIX modifies "domain", not "domain socket"; "UNIX domain" modifies "socket", and "domain socket" by itself is meaningless.

This matches the phrasing and capitalization from POSIX.1:

http:https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_10_17

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I looked at some other recently submitted PRs, and this comment suggested running "make docs" after a doc change, but when I did that it resulted in some extraneous whitespace changes in doc/stdlib/parallel.rst.

[tkelman]

bit redundant to say it twice?

[StefanKarpinski]

Do we support similar functionality on non-POSIX systems like Windows? If so, we should use more generic terminology and give UNIX domain sockets as an example system-level implementation somewhere. cc @vtjnash, @Keno

[vtjnash]

The Windows name is Named Pipe, as given here

[sorear]

https://msdn.microsoft.com/en-us/library/windows/desktop/aa365590(v=vs.85).aspx

A named pipe is a named, one-way or duplex pipe for communication between the pipe server and one or more pipe clients. All instances of a named pipe share the same pipe name, but each instance has its own buffers and handles, and provides a separate conduit for client/server communication. The use of instances enables multiple pipe clients to use the same named pipe simultaneously.

AFAICT it's "named pipe", not "Named Pipe".