docs: document socket.unix and socket.serial, fix stale/missing API referencesdocs/pre-release-audit

Pre-release documentation audit against master. Adds the two fully
undocumented modules and closes gaps found by diffing every docs/*.html
page against its corresponding source:

- New docs/unix.html and docs/serial.html (socket.unix stream/dgram and
  socket.serial were never documented), linked from index.html,
  introduction.html, socket.html and reference.html. serial.html notes
  the current lack of a baud/parity/flow-control API and includes an
  os.execute+stty workaround example. Both note Windows is unsupported.
- socket.html: document headers.setcanonic, point to unix/serial modules.
- tcp.html: document getfamily, setpeername/setsockname aliases.
- udp.html: document getfamily, getfd/setfd, dirty; document the
  ipv6-multicast-hops/ipv6-unicast-hops aliasing as current (known-buggy)
  behavior rather than the originally intended semantics.
- http.html: correct redirect text (301/302/303/307/308, was 301/302
  only); document MAXHEADERLINE/MAXHEADERSIZE.
- dns.html: document getnameinfo.
- ltn12.html: document source.rewind and BLOCKSIZE.
- installation.html: fix stale "LuaSocket 3.0" sample output to 3.1.0.
- reference.html: index every anchor added above plus new Unix/Serial
  blocks.

1 files changed, 4 insertions, 1 deletions

diff --git a/docs/introduction.html b/docs/introduction.html
index 51443a0..bc0d535 100644
--- a/docs/introduction.html
+++ b/docs/introduction.html
@@ -304,7 +304,10 @@ io.write(assert(udp:receive()))
 <p> Although not covered in the introduction, LuaSocket offers
 much more than TCP and UDP functionality. As the library
 evolved, support for <a href=http.html>HTTP</a>, <a href=ftp.html>FTP</a>,
-and <a href=smtp.html>SMTP</a> were built on top of these. These modules
+and <a href=smtp.html>SMTP</a> were built on top of these. On Unix-like
+platforms, the library also ships separate <a href=unix.html>Unix domain
+socket</a> and <a href=serial.html>serial port</a> modules, requirable as
+<tt>socket.unix</tt> and <tt>socket.serial</tt>. These modules
 and many others are covered by the <a href=reference.html>reference manual</a>.
 </p>