Don't confuse users with http+unix:// and URL-encoding socket path at all

In 5c66f954 (Allow to configure gitlab socket just by path) we added
gitlab_socket option to allow users to configure path to Unicorn socket
just by path.

But @jacobvosmaer asks for even more - make http+unix:// stuff internal
and not even try to confuse users with e.g. description of URL-encoding
socket path and possibility to configure sockets with gitlab_url and
just direct unconditionally to gitlab_socket for that case.

So be it - for users it is now shown that the only way to configure
sockets is via gitlab_socket option. Internally we still use
http+unix:// scheme and handle it even if it is given in gitlab_url.

config.yml.example

@@ -11,18 +11,12 @@ user: git
 # You only have to change the default if you have configured Unicorn
 # to listen on a custom port, or if you have configured Unicorn to
 # only listen on a Unix domain socket. For Unix domain sockets use
-# "http+unix://<urlquoted-path-to-socket>/", e.g.
-# "http+unix://%2Fpath%2Fto%2Fsocket/"
-#
-# see also: gitlab_socket
+# gitlab_socket option.
 gitlab_url: "http://localhost:8080/"
 
 # Path to unix socket for gitlab instance.
-# Can be used instead of http+unix://... if one is not comfortable with
-# URL-encoding socket path in gitlab_url. Only one of gitlab_url or
-# gitlab_socket can be non-empty. Default to ""
-#
-# see also: gitlab_url
+# Should be used instead of gitlab_url option if you configured Unicorn to
+# listen on a Unix domain socket.
 gitlab_socket: ""
 
 # See installation.md#using-https for additional HTTPS configuration details.