Auto merge of #34739 - therealbstern:ipv4unspec, r=alexcrichton

Auto merge of #34739 - therealbstern:ipv4unspec, r=alexcrichton

Mark Ipv4Addr is_unspecified as stable and provide reference.

Per [#27709 (comment)](https://github.com/rust-lang/rust/issues/27709#issuecomment-231280999), no RFC is needed here.

IPv4 "unspecified" has been defined in [Stevens], and has been part of the IPv4 stack for quite some time.  This property should become stable, since this use of 0.0.0.0 is not going anywhere.

[Stevens][_UNIX Network Programming Volume 1, Second Edition_.  Stevens, W. Richard.  Prentice-Hall, 1998.  p. 891]

Please let me know if I got the rustdoc wrong or something.  I tried to be as terse as possible while still conveying the appropriate information.

This also has a slight impact on PR #34694, but that one came first, so this shouldn't block it, IMO.