request-pull: documentation updates

The original description talked only about what it does.  Instead,
start it with the purpose of the command, i.e. what it is used for,
and then mention what it does to achieve that goal.

Clarify what <start>, <url> and <end> means in the context of the
overall purpose of the command.

Describe the extended syntax of <end> parameter that is used when
the local branch name is different from the branch name at the
repository the changes are published.

Helped-by: Eric Sunshine
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-request-pull.txt

@@ -13,22 +13,65 @@ SYNOPSIS
 DESCRIPTION
 -----------
 
-Summarizes the changes between two commits to the standard output, and includes
-the given URL in the generated summary.
+Generate a request asking your upstream project to pull changes into
+their tree.  The request, printed to the standard output, summarizes
+the changes and indicates from where they can be pulled.
+
+The upstream project is expected to have the commit named by
+`<start>` and the output asks it to integrate the changes you made
+since that commit, up to the commit named by `<end>`, by visiting
+the repository named by `<url>`.
+
 
 OPTIONS
 -------
 -p::
-	Show patch text
+	Include patch text in the output.
 
 <start>::
-	Commit to start at.
+	Commit to start at.  This names a commit that is already in
+	the upstream history.
 
 <url>::
-	URL to include in the summary.
+	The repository URL to be pulled from.
 
 <end>::
-	Commit to end at; defaults to HEAD.
+	Commit to end at (defaults to HEAD).  This names the commit
+	at the tip of the history you are asking to be pulled.
++
+When the repository named by `<url>` has the commit at a tip of a
+ref that is different from the ref you have locally, you can use the
+`<local>:<remote>` syntax, to have its local name, a colon `:`, and
+its remote name.
+
+
+EXAMPLE
+-------
+
+Imagine that you built your work on your `master` branch on top of
+the `v1.0` release, and want it to be integrated to the project.
+First you push that change to your public repository for others to
+see:
+
+	git push https://git.ko.xz/project master
+
+Then, you run this command:
+
+	git request-pull v1.0 https://git.ko.xz/project master
+
+which will produce a request to the upstream, summarizing the
+changes between the `v1.0` release and your `master`, to pull it
+from your public repository.
+
+If you pushed your change to a branch whose name is different from
+the one you have locally, e.g.
+
+	git push https://git.ko.xz/project master:for-linus
+
+then you can ask that to be pulled with
+
+	git request-pull v1.0 https://git.ko.xz/project master:for-linus
+
 
 GIT
 ---