Documenting topic use case of push options

This change provides documentation of a particular use case for
push options, namely the setting of the topic strings used to
group related changes.

Change-Id: I16c0aac33825a86d4667cb4a10f746efd33203cb
Signed-off-by: Dan Wang <dwwang@google.com>

Documentation/intro-user.txt

@@ -470,11 +470,16 @@ Assigning a topic to a change can be done in the
 link:user-review-ui.html#project-branch-topic[change screen].
 
 It is also possible to link:user-upload.html#topic[set a topic on
-push].
+push], either by appending `%topic=...` to the ref name or through
+the use of the command line flag `--push-option`, aliased to `-o`,
+followed by `topic=...`.
 
 .Set Topic on Push
 ----
   $ git push origin HEAD:refs/for/master%topic=multi-master
+
+  // this is the same as:
+  $ git push origin HEAD:refs/heads/master -o topic=multi-master
 ----
 
 [[drafts]]

Documentation/user-upload.txt

@@ -168,12 +168,16 @@ By default all email notifications are sent.
 [[topic]]
 To include a short tag associated with all of the changes in the
 same group, such as the local topic branch name, append it after
-the destination branch name. In this example the short topic tag
+the destination branch name or add it with the command line flag
-'driver/i42' will be saved on each change this push creates or
+`--push-option`, aliased to `-o`. In this example the short topic
+tag 'driver/i42' will be saved on each change this push creates or
 updates:
 
 ----
   git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/experimental%topic=driver/i42
+
+  // this is the same as:
+  git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/experimental -o topic=driver/i42
 ----
 
 [[message]]