doc: git-push: clarify "where to push"

It's not obvious that "`branch.*.remote` configuration"` refers to the
upstream, so say "upstream" instead.

The sentence is also quite hard to parse right now, use "defaults to" to
simplify it.

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

Author: jvns

Documentation/git-push.adoc

@@ -23,10 +23,8 @@ 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.
 
-When the command line does not specify where to push with the
-`<repository>` argument, `branch.*.remote` configuration for the
-current branch is consulted to determine where to push.  If the
-configuration is missing, it defaults to 'origin'.
+The `<repository>` argument defaults to the upstream for the current branch,
+or `origin` if there's no configured upstream.
 
 When the command line does not specify what to push with `<refspec>...`
 arguments or `--all`, `--mirror`, `--tags` options, the command finds