Fixes and documentation for teardown and use with SSL:

This solves a problem where clang and gcc locate the deleted
version of teardown and async_teardown instead of the overloaded
version. It requires overloads to add `teardown_tag` into the signature
so that the rules for argument dependent lookup can find the
right function. Improve documentation of teardown requirements

The documentation is updated to clearly explain the need for including
<beast/websocket/ssl.hpp> to use SSL streams with WebSocket.

The default implementations of teardown and async_teardown now use
static_assert to alert the user of improper usage, with comments
providing guidance for resolving the error.

include/beast/websocket/impl/ssl.ipp

@@ -129,7 +129,7 @@ operator()(error_code ec, bool again)
 
 template<class AsyncStream>
 void
-teardown(
+teardown(teardown_tag,
     boost::asio::ssl::stream<AsyncStream>& stream,
         error_code& ec)
 {
@@ -138,7 +138,7 @@ teardown(
 
 template<class AsyncStream, class TeardownHandler>
 void
-async_teardown(
+async_teardown(teardown_tag,
     boost::asio::ssl::stream<AsyncStream>& stream,
         TeardownHandler&& handler)
 {

include/beast/websocket/impl/teardown.ipp

@@ -128,7 +128,7 @@ operator()(error_code ec, std::size_t, bool again)
 
 inline
 void
-teardown(
+teardown(teardown_tag,
     boost::asio::ip::tcp::socket& socket,
         error_code& ec)
 {
@@ -151,7 +151,7 @@ teardown(
 template<class TeardownHandler>
 inline
 void
-async_teardown(
+async_teardown(teardown_tag,
     boost::asio::ip::tcp::socket& socket,
         TeardownHandler&& handler)
 {