docs: Add branch tracking to branches.md.

This tries to explain Jujutsu's branch tracking for a newcomer. 
It is based on it's design doc in `docs/design/tracking-branches.md`.

docs/branches.md

@@ -11,7 +11,6 @@ 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 identifies a branch by its name across remotes (this is unlike Git and
@@ -41,6 +40,70 @@ branch `main`. If the local target had also moved compared to `main@origin`
 merged. If one is ahead of the other, then that target will be the new target.
 Otherwise, the local branch will be conflicted (see next section for details).
 
+<!-- TODO: Adjust this paragraph to the new defaults which were introduced in #2736 -->
+As of December 2023 Jujutsu tracks[^1] and fetches all branches by default, 
+which is confusing users coming from Git. To smoothen the transition branch 
+tracking was introduced. 
+
+### What does `git.auto-local-branch` actually do? 
+
+Jujutsu's fetch operations consist of several steps. First `jj git fetch`
+fetches all Git refs under `/refs/remotes/origin` (or, if you have 
+multiple remotes `/refs/remotes/<remote name>` for each remote).  
+Then Jujutsu stores these refs as remote tracking branches. Finally, by default, 
+Jujutsu creates local branches for them. This is similar to Mercurial, which 
+fetches all it's Booksmarks (equivalent to Git branches) by default. 
+
+There are two ways to disable the creation (or modification) of the local 
+branches by `jj git fetch`: 
+
+ * You can use `jj branch untrack <branch-name>@<remote name>` to stop tracking
+ specific branches when fetching from specific remotes. 
+ * You can set `git.auto-local-branch = false` to change the default behavior.
+ Then, Jujutsu will only create local branches for remote branches which you 
+ explicitly track with `jj branch track<branch name>@<remote name>`.
+
+### 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
+on the remote. 
+
+Example:
+
+```sh
+$ # List all available branches, as we want our colleague's branch.
+$ jj branch list --all
+$ # Find the branch.
+$ # [...]
+$ # Actually track the branch.
+$ jj branch track <branch name>@<remote name> # Example: jj branch track my-feature@origin
+$ # From this point on, branch <name> is tracked and will always be imported.
+$ jj git fetch # Update the repository
+$ jj new <name> # Do some local testing, etc.
+```
+
+### Untracking a branch
+
+To no longer have a branch available in a repository, you can 
+`jj branch untrack` it. After that subsequent fetches will no longer copy the 
+branch into the local repository. 
+
+Example: 
+
+```sh
+$ # List all local and remote branches.
+$ jj branch list --all
+$ # Find the branch we no longer want to track.
+$ # [...]
+# # Actually untrack it.
+$ jj branch untrack <branch name>@<remote name> # Example: jj branch untrack stuff@origin
+$ # 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].
+
 
 ## Conflicts
 
@@ -65,3 +128,6 @@ on top of the other with `jj rebase`.
 To resolve a conflicted state in a remote branch (e.g. `main@origin`), simply
 pull from the remote (e.g. `jj git fetch`). The conflict resolution will also
 propagate to the local branch (which was presumably also conflicted).
+
+[^1]: Tracking in this context means if `jj` should create a local branch for a remote branch.
+[design]: design/tracking-branches.md