manual: Extend documentation about subset rebase with git equivalent

The description of `magit-rebase-subset' did not mention the underlying
Git command it runs, nor does it explain that HEAD must already be at
the desired upper bound before invoking it.

This is a discoverability problem for users coming from the Git CLI.
"git rebase --onto NEWBASE START END" is a well-known pattern, but
there was no hint in the manual that this is its Magit equivalent.

1 files changed, 4 insertions, 0 deletions

diff --git a/docs/magit.org b/docs/magit.org
index b24e1bb..98ccbad 100644
--- a/docs/magit.org
+++ b/docs/magit.org
@@ -6094,6 +6094,10 @@ Remotes]].
   commits from START to ~HEAD~ onto NEWBASE.  START has to be selected
   from a list of recent commits.
 
+  This command calls ~git rebase --onto NEWBASE START HEAD~.  Because
+  the range always ends at ~HEAD~, you must first check out the branch
+  or commit you want as the upper bound (a detached ~HEAD~ works too).
+
 By default Magit uses the ~--autostash~ argument, which causes
 uncommitted changes to be stored in a stash before the rebase begins.
 These changes are restored after the rebase completes and if possible