docs branches.md: explain the new defaults and reorganize

This follows up on jj-vcs#2625 and updates all sections of `branches.md` to represent
the new default of `git.auto-local-branch=false`.

I also realized that the paragraph we discussed for so long with @PhilipMetzger
actually repeats information that was already present a bit earlier in the file.
So, I removed most of it and moved the rest. Sorry I didn't notice this earler.

docs/branches.md

@@ -11,23 +11,28 @@ pass a branch's name to commands that want a revision as argument. For example,
 `jj branch list` to list branches and `jj branch` to create, move, or delete
 branches. There is currently no concept of an active/current/checked-out branch.
 
-## Remotes
-
-Jujutsu records the last seen position on each remote (just like Git's
-remote-tracking branches). You can refer to these with
-`<branch name>@<remote name>`, such as `jj new main@origin`.
-
-A remote branch can be associated with a local branch of the same name. It's
-sometimes called a tracking branch. When you pull from a remote, any changes
-compared to the current record of the remote's state will be propagated to the
-tracking local branch. Let's say you run `jj git fetch --remote origin` and the
-remote's "main" branch has moved so its target is now ahead of the local record
-in `main@origin`. That will update `main@origin` to the new target. It will also
-apply the change to the local branch `main`. If the local target had also moved
-compared to `main@origin` (probably because you had run `jj branch set main`),
-then the two updates will be merged. If one is ahead of the other, then that
-target will be the new target. Otherwise, the local branch will be conflicted
-(see the next "Conflicts" section for details).
+## Remotes and tracking branches
+
+Jujutsu records the last seen position of a branch on each remote (just like
+Git's remote-tracking branches). This record is updated on every `jj git fetch`
+and `jj git push` of the branch. You can refer to the remembered remote branch
+positions with `<branch name>@<remote name>`, such as `jj new main@origin`. `jj`
+does not provide a way to manually edit these records.
+
+A remote branch can be associated with a local branch of the same name. This is
+called a **tracked branch**. When you pull a tracking branch from a remote, any
+changes compared to the current record of the remote's state will be propagated
+to the tracking local branch.
+
+**Example and details:** Let's say you run `jj git fetch --remote origin` and
+the remote's "main" branch has moved so its target is now ahead of the local
+record in `main@origin`. That will update `main@origin` to the new target. If
+`main@origin` is **tracked**, `jj` will also apply the change to the local
+branch `main`. If the local target had also moved compared to `main@origin`
+(probably because you had run `jj branch set main`), then the two updates will
+be merged. If one is ahead of the other, then that target will be the new
+target. Otherwise, the local branch will be conflicted (see the ["Conflicts"
+section](#conflicts) below for details).
 
 Most commands don't show the tracking remote branch if it has the same target as
 the local branch. The local branch (without `@<remote name>`) is considered the
@@ -38,23 +43,10 @@ target there, `jj log` will show the branch name with an asterisk suffix
 (e.g. `main*`). That is meant to remind you that you may want to push the branch
 to some remote.
 
-By default, the default remote branch (e.g. `main@origin`) will be tracked
-automatically. You can use `jj branch track` to track existing remote branches
-individually, or set `git.auto-local-branch = true` configuration to track all
-new remote branches automatically.
-
-### What does `git.auto-local-branch = true` actually do?
-
-Jujutsu's fetch operation consist of several steps. First `jj git fetch` fetches
-all Git refs under `refs/remotes/<remote name>`. Then Jujutsu stores these refs
-as remote tracking branches. Finally, if `git.auto-local-branch = true`, Jujutsu
-creates local branches for them. This is similar to Mercurial, which fetches all
-its bookmarks (equivalent to Git branches) by default.
-
-You can use `jj branch untrack <branch name>@<remote name>` to stop tracking
-specific branches when fetching from specific remotes.
+If you want to know the internals of branch tracking, consult the 
+[Design Doc][design].
 
-### Tracking a branch
+### Manually tracking a branch
 
 To track a branch permanently use `jj branch track <branch name>@<remote name>`. 
 It will now be imported as a local branch until you untrack it or it is deleted
@@ -92,8 +84,16 @@ $ jj branch untrack <branch name>@<remote name> # Example: jj branch untrack stu
 $ # From this point on, it won't be imported anymore. 
 ```
 
-If you want to know the internals of branch tracking, consult the 
-[Design Doc][design].
+### Automatic tracking of branches & `git.auto-local-branch` option
+By default, after a `jj git clone`, the default remote branch (e.g.
+`main@origin`) is tracked automatically. Every other branch is marked as "not
+tracked" when it's created.
+
+This default can be changed by setting the config `git.auto-local-branch =
+true`. Then, `jj git fetch` marks every *new* branch as "tracked". Branches that
+already existed before the `jj git fetch` are not affected. This is similar to
+Mercurial, which fetches all its bookmarks (equivalent to Git branches) by
+default.
 
 
 ## Conflicts