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.