doc: git-push: Add explanation of git push origin main

What happens if you run `git push` without any arguments is actually
extremely complex to explain, as discussed in the previous commit.

But it's very easy to explain what `git push <remote> <branch>` does, so
start the man page by explaining what that does.

The hope is that someone could just stop reading the man page here and
never learn anything else about `git push`, and that would be fine.

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

Author: jvns

Documentation/git-push.adoc

@@ -23,6 +23,10 @@ Updates one or more branches, tags, or other references in a remote
 repository from your local repository, and sends all necessary data
 that isn't already on the remote.
 
+The simplest way to push is `git push <remote> <branch>`.
+`git push origin main` will push the local `main` branch to the `main`
+branch on the remote named `origin`.
+
 The `<repository>` argument defaults to the upstream for the current branch,
 or `origin` if there's no configured upstream.