rebase: tweak description of inmemory working even w/ dirty working dir

One of our users was confused because they read this, and then attempted to run
`hg rebase` with a dirty working directory, and it still complained. The reason
was that they were attempting to rebase the commit they currently had checked
out, which (at least with evolve workflows enabled) involves updating the
working directory to be based on the newly rebased commit.

Differential Revision: https://phab.mercurial-scm.org/D6507

hgext/rebase.py

@@ -850,8 +850,9 b' def rebase(ui, repo, **opts):'
       singletransaction = True
 
     By default, rebase writes to the working copy, but you can configure it to
-    run in-memory for for better performance, and to allow it to run if the
-    working copy is dirty::
+    run in-memory for better performance. When the rebase is not moving the
+    parent(s) of the working copy (AKA the "currently checked out changesets"),
+    this may also allow it to run even if the working copy is dirty::
 
       [rebase]
       experimental.inmemory = True