Documentation/urls: Rewrite to accomodate <transport>::<address>

Rewrite the first part of the document to explicitly show differences
between the URLs that can be used with different transport
protocols. Mention <transport>::<address> format to explicitly invoke
a remote helper.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/urls.txt

@@ -1,44 +1,57 @@
 GIT URLS[[URLS]]
 ----------------
 
-One of the following notations can be used
-to name the remote repository:
+In general, URLs contain information about the transport protocol, the
+address of the remote server, and the path to the repository.
+Depending on the transport protocol, some of this information may be
+absent.
+
+Git natively supports ssh, git, http, https, ftp, ftps, and rsync
+protocols. The following syntaxes may be used with them:
 
-- rsync://host.xz/path/to/repo.git/
-- http://host.xz{startsb}:port{endsb}/path/to/repo.git/
-- https://host.xz{startsb}:port{endsb}/path/to/repo.git/
-- git://host.xz{startsb}:port{endsb}/path/to/repo.git/
-- git://host.xz{startsb}:port{endsb}/~user/path/to/repo.git/
 - ssh://{startsb}user@{endsb}host.xz{startsb}:port{endsb}/path/to/repo.git/
-- ssh://{startsb}user@{endsb}host.xz/path/to/repo.git/
-- ssh://{startsb}user@{endsb}host.xz/~user/path/to/repo.git/
-- ssh://{startsb}user@{endsb}host.xz/~/path/to/repo.git
+- git://host.xz{startsb}:port{endsb}/path/to/repo.git/
+- http{startsb}s{endsb}://host.xz{startsb}:port{endsb}/path/to/repo.git/
+- ftp{startsb}s{endsb}://host.xz{startsb}:port{endsb}/path/to/repo.git/
+- rsync://host.xz/path/to/repo.git/
 
-SSH is the default transport protocol over the network.  You can
-optionally specify which user to log-in as, and an alternate,
-scp-like syntax is also supported.  Both syntaxes support
-username expansion, as does the native git protocol, but
-only the former supports port specification. The following
-three are identical to the last three above, respectively:
+An alternative scp-like syntax may also be used with the ssh protocol:
 
-- {startsb}user@{endsb}host.xz:/path/to/repo.git/
-- {startsb}user@{endsb}host.xz:~user/path/to/repo.git/
-- {startsb}user@{endsb}host.xz:path/to/repo.git
+- {startsb}user@{endsb}host.xz:path/to/repo.git/
 
-To sync with a local directory, you can use:
+The ssh and git protocols additionally support ~username expansion:
+
+- ssh://{startsb}user@{endsb}host.xz{startsb}:port{endsb}/~{startsb}user{endsb}/path/to/repo.git/
+- git://host.xz{startsb}:port{endsb}/~{startsb}user{endsb}/path/to/repo.git/
+- {startsb}user@{endsb}host.xz:/~{startsb}user{endsb}/path/to/repo.git/
+
+For local respositories, also supported by git natively, the following
+syntaxes may be used:
 
 - /path/to/repo.git/
 - file:///path/to/repo.git/
 
 ifndef::git-clone[]
-They are mostly equivalent, except when cloning.  See
-linkgit:git-clone[1] for details.
+These two syntaxes are mostly equivalent, except when cloning, when
+the former implies --local option. See linkgit:git-clone[1] for
+details.
 endif::git-clone[]
 
 ifdef::git-clone[]
-They are equivalent, except the former implies --local option.
+These two syntaxes are mostly equivalent, except the former implies
+--local option.
 endif::git-clone[]
 
+When git doesn't know how to handle a certain transport protocol, it
+attempts to use the 'remote-<transport>' remote helper, if one
+exists. To explicitly request a remote helper, the following syntax
+may be used:
+
+- <transport>::<address>
+
+where <address> may be a path, a server and path, or an arbitrary
+URL-like string recognized by the specific remote helper being
+invoked. See linkgit:git-remote-helpers[1] for details.
 
 If there are a large number of similarly-named remote repositories and
 you want to use a different format for them (such that the URLs you