doc: git rebase: clarify arguments syntax

I found this extremely hard to understand as it was and this seemed like
a clearer wording to me.

Signed-off-by: Julia Evans <[email protected]>

Author: jvns

Documentation/git-rebase.adoc

@@ -59,12 +59,10 @@ one of these things:
 
    git rebase --skip
 
+`git rebase <upstream> <branch>` is shorthand for `git switch <branch>`
+followed by `git rebase <upstream>`.
 
-If `<branch>` is specified, `git rebase` will perform an automatic
-`git switch <branch>` before doing anything else.  Otherwise
-it remains on the current branch.
-
-If `<upstream>` is not specified, the upstream configured in
+If you don't specify an `<upstream>` to rebase onto, the upstream configured in
 `branch.<name>.remote` and `branch.<name>.merge` options will be used (see
 linkgit:git-config[1] for details) and the `--fork-point` option is
 assumed.  If you are currently not on any branch or if the current