Reword historical commit instructions

git/git-lfs.rst

@@ -144,11 +144,18 @@ For instance, if you were working with
 
 .. _git-lfs-historical:
 
-Checking out Historical Commits
+Checking out historical commits
 ===============================
 
-If you want to check out a historical commit, you will find that the LFS
-object fetch fails, because only recent commits will reference
+If you want to check out a historical commit, you first need to know
+that arbitrary commits are no longer available.  When we migrated from
+``git-lfs.lsst.codes`` to ``git-lfs.lsst.cloud`` we only migrated LFS
+objects that were either at the tip of the ``main`` branch or a release
+branch (one whose name begins with ``v`` followed by a digit), or were
+referenced in a Git tag.
+
+If your proposed checkout meets these criteria, next you will will find
+that the LFS object fetch fails, because only recent commits reference
 ``git-lfs.lsst.cloud`` rather than ``git-lfs.lsst.codes``, and the
 checkout will reset ``.lfsconfig`` to its old value.  What you will need
 to do in that case is the following.