docs: Add a short section on branch movement.

This is something which should've been written down long ago. This is a follow-up upon 
another question from Julia Evans in Discord. 

Thank you again for asking such good questions.

docs/branches.md

@@ -138,6 +138,17 @@ 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.
 
+## Branch movement
+
+Currently Jujutsu only moves branches by itself when these conditions are met:
+
+ * When a commit has been rewritten (so when you `jj split` a change) the 
+   working-copy will move along with it.
+ * When a commit has been abandoned, all associated branches will with the 
+   working-copy to its parent(s).
+
+You could describe the movement as following along the change-id of the 
+working-copy commit, even if it isn't entirely accurate.
 
 ## Conflicts