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

From user feedback: 6 users says they found the "what to push"
paragraphs confusing, for many different reasons, including:

* what does "..." in <refspec>... mean?
* "consult XXX configuration" is hard to parse
* it refers to the `git-config` man page even though the config
  information for `git push` is included in this man page under
  CONFIGURATION
* the default ("push to a branch with the same name") is what they use
  99% of the time, they would have expected it to appear earlier instead
  of at the very end
* not understanding what the term "upstream" means in Git
  ("are branches tracked by some system besides their names?"")

Also, the current explanation of `push.default=simple` ("the
current branch is pushed to the corresponding upstream branch, but
as a safety measure, the push is aborted if the upstream branch
does not have the same  name as the local one.") is not accurate:
`push.default=simple` does not always require you to set a corresponding
upstream branch.

Address all of these by

* using a numbered "in order of precedence" list
* giving a more accurate explanation of how `push.default=simple` works
* giving a little bit of context around "upstream branch": it's
  something that you may have to set explicitly
* referring to the new UPSTREAM BRANCHES section

The default behaviour is still discussed pretty late but it should be
easier to skim now to get to the relevant information.

In "`git push` may fail if...",  I'm intentionally being vague about
what exactly `git push` does, because (as discussed on the mailing list)
the behaviour of `push.default=simple` is very confusing, perhaps broken,
and certainly not worth trying to explain in an introductory context.
`push.default.simple` sometimes requires you to set an upstream and
sometimes doesn't and the exact conditions under which it does/doesn't
are hard to describe.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-push.adoc

@@ -26,18 +26,20 @@ that isn't already on the remote.
 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
-the default `<refspec>` by consulting `remote.*.push` configuration,
-and if it is not found, honors `push.default` configuration to decide
-what to push (See linkgit:git-config[1] for the meaning of `push.default`).
+To decide which branches, tags, or other refs to push, Git uses
+(in order of precedence):
 
-When neither the command-line nor the configuration specifies what to
-push, the default behavior is used, which corresponds to the `simple`
-value for `push.default`: the current branch is pushed to the
-corresponding upstream branch, but as a safety measure, the push is
-aborted if the upstream branch does not have the same name as the
-local one.
+1. The `<refspec>` argument(s) (for example `main` in `git push origin main`)
+   or the `--all`, `--mirror`, or `--tags` options
+2. The `remote.*.push` configuration for the repository being pushed to
+3. The `push.default` configuration. The default is `push.default=simple`,
+   which will push to a branch with the same name as the current branch.
+   See the <<CONFIGURATION,CONFIGURATION>> section below for more on `push.default`.
+
+`git push` may fail if you haven't set an upstream for the current branch,
+depending on what `push.default` is set to.
+See the <<UPSTREAM-BRANCHES,UPSTREAM BRANCHES>> section below for more
+on how to set and use upstreams.
 
 You can make interesting things happen to a repository
 every time you push into it, by setting up 'hooks' there.  See
@@ -702,6 +704,7 @@ a `git gc` command on the origin repository.
 
 include::transfer-data-leaks.adoc[]
 
+[[CONFIGURATION]]
 CONFIGURATION
 -------------